So long as you have plenty of overlap all around for the seal, it should be fine. Just make sure you hold the trap as central as possible while you tighten it up. It's like taps and tap holes, invariably the holes are far bigger than the tap stems.

I would advise checking the smoothness of the underside of the tray where the trap seals and sand flat if necessary. Also it might be prudent to use some silicon to aid with the sealing underneath. The last thing you want is a leaking shower base once it's all in and fixed

hi thx for your replies guys i have posted a pic of the type of trap i am wanting to fit basically the the part which screw in from the top has a diametre of approx 40mm this is threaded part not the top flange and the silver top part doesn,t seem to seeat as it should i will try post pic of this later if i can sorry thats the wrong pic

correct picture here