What?

If you had the receipt then surely you can prove beyond all doubt it's yours? The bike shop you got it from will have logged the frame number so the basis for their reasoning is shakey to say the least.

End of the day thats what police complaints are there for, that'll soon get them paying attention along with press coverage if needed...

if your using your bike for commuting, (not riding to mtb spots) I can strongly reccomend the awesomess of having an appropriate bike.

Being able to ride 20-30mph consistently without getting tired is a real joy. You feel so much safer as you can keep up with traffic,  use all lanes/roundabouts and pull away out of sticky situations. Its also not so annoying when it rains.

If im on a BMX I stick to the pavement mostly as drivers dont give you much respect on the road, but it still feels rude to ride on the pavement.

Go down to the station and they should be able to help you, Unfortunately you will need their badge number for it to be of any use IIRC

Was the previous owner the original? If so they should be able to get in contact with where they got the bike and get a confirmation of the frame number.

Good luck anyway...