51 reg is right on the cusp of 2/2.5. You can tell by the headlights.
Lights on mine are the segmented lights so it’s a 2.5 
Thanks. Sounds pretty much the same as yours…
Possibly means the spare key cut is best done at a dealer though…
The definitive answer is here.
Superficially MK2s have a single large multi-filament headlight + Indicator while the MK2.5s have segmented lights with multiple single filaments.
I’d suggest that if you don’t have a special edition you are likely to be able to go down the Timpsons route (don’t know 'cos I haven’t tried it).
Whatever it costs a second key is not a luxury. Get it sorted soonest. If Timpsons can’t programme the key, a dealer visit is the only option. My 2003 Nevada had to be programmed at a dealer (John Pease in Chelmsford). They supplied the key, cut it to spec using the number I supplied, and programmed the car, new key and old key within an hour or two for £70 (including MX5 OC discount).
Pretty much What I did, rang John Pease, and a few local, Docklands, Thurrock, John Pease wanted 130 all in. My local was 120. 60 for the key, and the rest on programming.
Picking it up Sat, and the 120 will hurt, but will hurt a lot more if I ever lose the key, so is a must IMO. Couldnt give me OC discount on the key, but the programming is done by the servicing department, the key itself was order via the parts department. Seprate entities within the dealer. Hope I get a discount on the programming. John Pease said they couldnt on keys???
No remote though, dealer confirmed on my model, so went out and got a Viper CAT 1 pager alarm, which is impressive alone. That hurt the wallet fitted also.
Spoke to a guy at my local dealers, he tells me supply them the vin no and they will get a key cut to order - The programming is done in the dealership after you get the key. Been told all in it’s around the £120 mark.
Got to be done - I will be having words with the garage I bought the car from and telling them I want the cost back from them. 
Spot on, had to take my v5 and second evidence of address so took my driving license.
Didnt even look at the key, just asked if any of the barrells have been changed, said I wasnt aware of so, my one key works in all and looks to be original. Key comes cut based on my chasis number, Key came in today actually, picking it up Sat morning, which it will then be programmed at the dealer.