Sounds like there should be a small nut and bolt which connects the two, you didn't mention if the loco was brand new or second hand, I suspect the latter. It's possible they have simply come loose and are now somewhere where they shouldn't be, so check carefully around the wheels and motor, the motor has magnets, so they attract all sorts of stuff you don't want near the motor.
Failing that, Peters Spares would be a good place to find the replacement parts, or you can make your own.

The two holes in the drawbar are so you can either have a close coupling between the two, which is fine on R3 or R4 Hornby track, or a looser coupling for tighter curves, such as the curved R2 or R1 Hornby track.

 

There is a small flat metal bar on the loco with two holes. On the tender there is a post with a tiny hole in it, and I guess the bar goes over this and is secured with a little screw and washer.

There's no securing screw, let alone a washer. This applies to all the Bachmann tender locos with this arrangement, introduced from 1999 to about 2010. (The SR N class produced slightly earlier had a camming link which was very poor, and this was dropped for the style you have seen.)

Bachmann made a mess of the 9F drawbar which spaced loco and tender at 'Grand Canyon' and 'Thames valley' distances; not their best effort on an otherwise very good loco model. It's easy to drill a hole for better spacing...

(Then they moved on using the same style drawbar but with one hole, engaging a peg under the tender, spacing adjustable with a screw locked slide. First class job, best loco to tender link in RTR OO.
Then they majorly boobed by adopting the now fashionable camming linkage on the recent V2 release, very poor indeed, Hopefully they will drop this like a brick, and go back to the previous class act...)

 

Sounds like there should be a small nut and bolt which connects the two, you didn't mention if the loco was brand new or second hand, I suspect the latter. It's possible they have simply come loose and are now somewhere where they shouldn't be, so check carefully around the wheels and motor, the motor has magnets, so they attract all sorts of stuff you don't want near the motor.
Failing that, Peters Spares would be a good place to find the replacement parts, or you can make your own.

The two holes in the drawbar are so you can either have a close coupling between the two, which is fine on R3 or R4 Hornby track, or a looser coupling for tighter curves, such as the curved R2 or R1 Hornby track.

Brand new. It seems from another reply there is no fixing it just goes over the peg.

There's no securing screw, let alone a washer. This applies to all the Bachmann tender locos with this arrangement, introduced from 1999 to about 2010. (The SR N class produced slightly earlier had a camming link which was very poor, and this was dropped for the style you have seen.)

Bachmann made a mess of the 9F drawbar which spaced loco and tender at 'Grand Canyon' and 'Thames valley' distances; not their best effort on an otherwise very good loco model. It's easy to drill a hole for better spacing...

(Then they moved on using the same style drawbar but with one hole, engaging a peg under the tender, spacing adjustable with a screw locked slide. First class job, best loco to tender link in RTR OO.
Then they majorly boobed by adopting the now fashionable camming linkage on the recent V2 release, very poor indeed, Hopefully they will drop this like a brick, and go back to the previous class act...)

Thanks for the comprehensive reply. The dates are interesting as i bought it new in 2021! The sliding idea is good, is there a picture anywhere as I have the facilities to possibly make one?

 

The dates are interesting as i bought it new in 2021!...

Bachmann haven't been much for 'updating' models, and run with the tooling as first introduced; in the case of the 9F, that came out in 2006, and is still the same TTBoMK.

The sliding idea is good, is there a picture anywhere as I have the facilities to possibly make one?

Had a quick search but cannot find one. It's simply a metal plate with a peg at the loco end and a slot down the middle, running in a guide moulded into the tender underside, and retained by a broad headed self tapper through the slot, which holds the selected position when screwed tight.

Personally, if required I simply cut off the fixed plastic peg, and use a self tapper as a substitute peg to adjust spacing to where I want it. There are obstacles often present to be aware of.

First that a steel ballast weight often lies above the location where the self tapper needs to go: tender top off and ease off the screws securing the ballast weight fixes that.

Secondly, Bachmann usually have moulded on intermediate buffers between loco and tender which may prevent positioning at scale distance, and/or prevent the loco getting around curves. As a practising vandal I just file these down as required, they cannot be seen easily when loco and tender are close coupled, which considerably ices the cake for appearance.

Thirdly, if a fall plate is present it may foul on tender front fittings such as brake stanchions, cutting clearance in the fall plate or slightly rearranging fittings as you feel best.

Fourthly, a high tender front or cab on the tender may foul on the loco cab when on curves and place a limit on how close to scale separation can be achieved.