Assuming this was factory-fitted by Hornby it will be one of theirs. The latest R8249 decoder is NMRA-compliant, and therefore should show an identifier of 48 if you read the manufacturer's value in CV8. But my M7 has the older R8215 decoder that doesn't seem to display an id. Instructions for both decoders are on the Hornby web-site - just search for R8215 or R8249. My M7 works fine with its R8215 as I only use 2-digit addresses.
Here's the R8215 link - http://static.hornby.com/files/r8215-loco-decoder-284.pdf
And the R8249 link - http://static.hornby.com/files/r8249-1228e...leaflet-423.pdf
Here's the R8215 link - http://static.hornby.com/files/r8215-loco-decoder-284.pdf
And the R8249 link - http://static.hornby.com/files/r8249-1228e...leaflet-423.pdf