Vaughan,
If you are paying real money for the Mini Glen, make sure you do a water ingress / damp test on all the inner panels - you can buy a cheap (but accurate) damp meter from DIY stores or Internet shops like www.screwfix.com
But if it's been stored in the dry, the damp meter might not reveal if it has leaked in the past.
Safety:
Tyres - everyone recommends changing them after 5 years and I'd try to get the tyres replaced before towing the van on the highway. Check the wheel rims for damage.
Chassis - I think the Thomson used the B&B chassis - just give it a look over underneath for rot or rust - should be clear. Also check out the underneath of the floor for problems.
Tow Coupling - check that it operates correctly, locks down OK, and that the brakeaway cable is in good condition. If you are in anyway unsure - replace these components - Towsure shops are good for this sort of stuff.
Brakes 1 - do a brake test, apply the handbrake (vertical position) and see if you can push the van forwards or backwards.
Release the handbrake and see if you can push the van forwards or backwards - after 9 years the brakes may have locked onto the drums.
Brakes 2 - on a private road - do a brake test with the Mini Glen coupled to the car - accelerate to 15 or 20 mph and then do an emergency stop. The caravan brakes should work and lock solid, then release as you drive away.
Brakes 3 - since at some stage you are going to have the wheels off - give the brakes a good look over. Consider brake shoe replacement, certainly a clean out with a brush - careful with the dust - and a careful (little) lubrication of the pivot points with a specialist brake lubricant like CopperSlip
Lighting - Check that the road lights work when coupled to your car - you'll probably need to do a bit of cleaning here.
Exterior - check for water tightness - windows and window rubbers, seals around aluminium trim. You may possibly need to remove some trim and reseal with new butyl mastic sealer.
Inside - then think about what you want or need to do on the inside.
Overall - This may read like it's years of work, but the safety checks and possible later rectification are less than 1/2 day to 1 day depending on what you need to do.
Book - Buy the Haynes manual on Caravan Repair and Servicing - maybe Halfords still have copies?
Sounds fun, Robert