What are the possible problems with pattern wishbones, i've heard the bushes are often slightly different sizes, making it tricky to fit poly bushes but are there any other problems?

Yes after market wishbones do vary in size when it comes to the bushes but most poly bush kits include enough parts to cover the different sizes.

Personally I'd go for Powerflex

I'd be more worried about the quality of the ball joint on some of the really cheap ones.

TRW (might even be OE I think???), Febi or Delphi are all good aftermarket brands for wishbones.

I've got TRW wishbones with powerflex bushes and they went on fine.

I got the noname QSA998S and QSA999S from Buypartsby.
The only fitting issue I found is that they're a bit thicker than OE so it is slightly more difficult to get the anti roll bar drop link bolts on. Not really too bad though. I've only done about 10k on them so too soon to decide if they're durable.

I've used the ones skinflint has and found both the ball joint covers split, which I replaced with OE ones from Fiat. This then meant I needed to get the tracking done again, adding cost. Also I found that the nuts which the bolts drive into strip easily.

I've since replaced the one which stripped a nut with a pattern part from AA, can't remember the make, but it was from Italy(!). So far so good.

Those are Quniton Hazell wishbones.

I used aftermarket ones (Delphi I think) and the car snaked alot and was very unstable - these inturn knackered the track rod ends - I ended up ordering OEM ones from alternative auto's along with polybushes - car has been great since.

When i renewed mine two years ago I used Original as my car had one aftermarket one fitted which I found when offered up to the new original was slightly different in size, i had noticed some differences before when id renewed my drop links

I had an aftermarket one fitted when I first bought the car and the balljoint "pin" was too thin; leading to lots of knocking on left turns. False economy to go with anything other than OEM IMO.

whats the prices on oem and copys ? surely the difference isnt that great to risk it ?

Depends on the OEM, I fitted delphi to my sprintie which were to all intents and purposes identical to the originals that I took off, albeit at less than half the price.