Update:
Fitted the new spark plugs, air filter & fuel filter and the problem got better but didn't go away completely, so spoke to my mate again and he told me (after confirming his thoughts with my other mate who I bought the bike from) that they recon it needs a new regulator rectifier, so found a new one on eBay for just under £40, it arrived today so I swapped out the old one and just got back from test riding and so far the problem has completely gone. So I'm now a happy bunny again.
Thanks everyone for all your helpful advice.