I'd start at
www.afors.co.uk, the PFA's website at
http://www.pfa.org.uk/the_pfa/sales_...fieds_list.asp or Popular Flying Magazine. No joy there, go to the PFA rally at the start of July, I saw a rough and ready MW7 there for I think £3500 which I was quite tempted by myself.
Insurance shouldn't be more than a few hundred pounds for 3rd party cover, last I looked hull insurance (not sure I'd bother on such an aircraft) would cost you around 8% of hull value per year. I personally wouldn't bother since generally the parts to repair most accidents will only be a few hundred pounds and PFA rules would allow you to do your own repairs.
If you can afford £4-£8k I'd also give John Hamer a call at the Minimax Club and see if there's a group A minimax going, they're usually around £5-6k and a much more refined aircraft than one or two single seaters I could mention. Alternatively the Rand KR2 is a cramped and unrefined but fast 2-seater that you should be able to pick up for under £8k.
G