Hi Guys, first of all I'm happy to report that I've managed to fix my lens so I'd like to say a massive thank you, especially to MysteryOnion who pinpointed the problem and Just1MoreDave - those photos were extremely useful and helped me 'see' problem and how to fix it.
Looks like the lens was taken apart at some point in it's history and when it was put back together they didn't fit the tiny ball bearing that sits between the body of the lens and the fstop selector ring. The one way resistance looks like it was purely coming from the washer/plate pushing against the aperture blade mechanism.
Thankfully I'm the type of person who keeps parts of things 'just in case they come in for something in the future' so after raiding my jar of ball bearings I was able to find a suitably sized one to fit ontop of the small spring. Re-assembled the lens and each fstop setting firmly clicks into place now, and stays there after I release my grip.
I only paid the equivalent of about 50usd for a ME super body, the lens mentioned above, a flash and camera bag and that included postage too, so I think trying to chase a refund for the lens would have been more effort than it was worth in this particular case, especially when you can get replacement lenses so cheaply on ebay, to be honest if I wasn't able to fix this myself then I'd have been more than happy to just throw it in my cupboard for spare parts and buy a replacement, but thanks to your advice I've been able to get it back fully functioning again.
Thanks once again guys, your help is much appreciated!
Cheers
Dan