Nick
What kind of car do you have? Firstly, there is no cat in the centre section, there is however a resonator. These can be replaced with a straight-through pipe and backbox with no need for a remap, thus the ECU is unaffected. Beyond the centre section there is cat pipe, this is the 3rd of three cats on the newage (I dunno anything about classics!). Again, this can be replaced with no need for remapping and will provide you with another 10bhp (so i'm told). Neither this nor the first suggestion will cause any problems with MOT or MPG for that matter. As mentioned, the up and down pipes have cats which can also be removed with a substantial power-gain (40 - 50 bhp?). The downside of course is MOT and ECU will be affected. A remap will cost you £500 or so, plus VAT then you will have to find a MOT Scoob-friendly centre
De-res centre and 3rd cat delete will cost you around the £200 mark worth every penny for the sound alone
Hope that helps.