Hornby West Country Short Circuit

I bought the Devon Belle Train Pack for my wife for her birthday, and have fitted an Hornby DCC decoder to it (not sure whether DCC relevant or not).
With just the Loco on the track it runs very well, but when we put the tender on the loco, it shorts out the whole system.
I can see 2 contacts on the eye on the loco which appear to contact a metal plate a the top of the pin on the tender, and I am guessing it is this that is shorting out the system. Should this be happening?
I am looking for advice as to
a) why it is happening and
what can be done to resolve this.

Hope someone can help

Thanks

Bill Bells

n.b. I have emailed Hornby, but their response time is usually not quick.