What you want is a Tipsy Nipper, your in luck as there is a very nice one for sale at the moment for £12500 which is an absolute bargain by the looks of it. Meets all of your criteria perfectly except the 2 seats!
1. Must be aerobatic and spin like a top, but semi-aerobatic is fine
-Check!, spins very well indeed and very aerobatic.
2. Must be on an LAA Permit to Fly - I'm an experienced RAF aircraft techie and have neither the inclination or funds(!) to pay someone else to do my routine maintenance.
-Check!
3. Ideally should have 2 seats, but a reasonably priced single seater would be a definite possibility
-Only 1 seat but very reasonable indeed.
4. Must have reasonably benign handling and good spin recovery characteristics - I'm fairly sure that a Pitts is a little beyond me for the moment!
-Check, Take your hands and feet off the controls and it'll pretty much sort itself out (as long as you watch the rear CG limit). Excellent handling and very responsive.
5. Should cost no more than £20K (but preferably quite a bit less)
-One for sale just now at £12,500
6. I fly from a huge concrete runway, but would really like a machine with good STOL performance for visiting farm strips
-I'm getting off the ground in about 50m with the Jabiru engine, VW's probably much the same, landing run is only a couple of hundred Meters.
7. I would prefer something which operates on MoGas but an AvGas burner isn't out of the question
-Check, Nippers can be run on MoGas or Avgas.
8. Would like something that can trundle along at about 60Kts if required so I can keep flying with my microlight buddies
-Cruises at 95mph but will happily fly at 60tks all day long. Stall is about 35kts.
9. Absolutely must have a stick not a yoke, and ideally a quadrant type throttle
-Check, stick and throttle.
Excellent owners forum can be found here:-
Tipsy Nipper • Index page
Aircraft for sale as follows (no its not mine as I wont be selling for a long time!):-
Tipsy Nipper • View topic - Nipper mk3
Mine costs me about £400 per year to insure, £20 per hour to fly on fuel and I do the maintenance myself which is pretty simple stuff.
With a climb rate of 1000fpm I can safely be doing aeros only 5mins after take off and typically back on the ground in about 30mins, a lot of fun for circa £10!
Plenty of vids on Youtube etc...