which magnex one should i need? resonator? cat, de-cat?

i have a MY95 uk spec. explain please what all these are, have a brief knowledge.

Cat/no cat:
Mid section cat is pretty pointless. By itself it most probably won't get you through an MOT, and if you have a cat in the DP, this will do it without the aid of the mid cat. So, pretty simple here - don't bother with the catted one!

Resonator/no resonator:
The resonator reduces noise level at a cost of losing (a very small amount of) efficiency. If you like quiet exhausts, get a resonator; if you like noisy, don't!

cheers, what do most people go for, i was told you can get a decat one put back in for a mot?>

You can wop a double-cat in for MOTs - ScoobySport & Subaru-4-You have them, and doubtless others do as well.
The double ones work 'cos the standard center has a secondary cat which just 'cleans up' what the primary leaves behind. The double center cat has the primary and a secondary, so works well, but takes longer to get going as it's further down the tube (longer to warm up). Center sections that do both but with minimal performance drop (they have a 'performance cat') are serious money. Well, fairly ;).