I'd appreciate a bit of fault finding advice please chaps.

A spot of background: not been using the car a huge amount over the winter, but have been using it a reasonable amount over the last few weeks. Always park up in gear, handbrake off to prevent calipers seizing.

Anyway, had a run last night (15 miles or so), first after a few days, no problem.

Drove to first site today (10 miles or so), all fine.

About an hour and a half later, drove to second site, noticed an acrid smell a couple of times so suspicions were aroused. When I parked up (3 miles?) the rear o/s wheel was so hot I couldn't get near it, lots of smoke pouring off it, acrid smell

I assumed the brakes were binding somehow, but too hot to do anything, so left in gear with h/b off.

Came back to the car an hour or so later. Car rolls fine, so brakes not binding. Drive a mile to next site, no problems.

After I have finished there, I removed the wheel to inspect.

Inside of wheel looks like a bin bag has melted on it, and then realisation dawns that it is the paint on the inside of the wheel (black) - it has blistered and pretty much come straight off

Again, hub turns freely, with just the normal amount of drag you would expect. Pads and discs look OK.

Can't see anything *except* what looks like a tiny weep from something on the back of the caliper. Brake fluid level fine though. CV boot clean and dry. Greasy/oily residue on inside sidewall of tyre

Wiped off as much of the bubbled/flaked paint as I could with turps, refitted wheel, on to next site (3-4 miles, no problem), and home (8 miles). No extreme heat from wheel (disc possibly slightly warmer than on n/s, but could still touch it for a second without burning).

So, thoughts?

As simple as a sticking caliper, making the wheel so hot that the paint melted, but has freed off now? Slight weep coincidental?

Or, did fluid leak blister and damage paint (only to the back of the wheel), but then why did it get so hot, and only on the one short journey?

Or, sticking caliper heats up wheel, melts grease in wheel bearing which coats tyre?

Any thoughts?

Couple of picture (excuse my thumb in the first )

Not sure how the handbrake works but the mech could be seizing on the calliper. More so because you don't use it much. Work it loose and lubricate.

Yup, caliber needs rebuilding/refreshing, it's just that it has freed itself offbefire your second journey. The grease will have cooked too on the sliders.

Checking the wheel bearing is a good call too.

Just got back from garage - caliper was weeping slightly as well as binding from time to time. New caliper and new brake fluid.

In the last mile or two I could hear some squealing from the rear o/s wheel, and it is hot to the touch after getting home.

bks.

What could be causing it? What can I check before I drive back to the garage?

Yes, I'd check the pads have been adjusted correctly.
Have a look here: http://www.mx5oc.co.uk/forum/forums/t/250.aspx
and here:
http://www.mx5oc.co.uk/forum/forums/t/254.aspx

Could it be a wheel bearing?

I can see the copper slip on the back of the pads! Wheel bearing not changed.

Just had a chance to get the wheel off - definitely too much drag on the wheel, I suspect the handbrake adjustment. Manual states both wheels off to adjust h/brake but I don't have enough room. Can I just adjust the side in question? TBH what I want to do is hook the cable off the caliper so it's just acting on the one wheel and get it back to him so he can do it properly as I don't trust myself with safety critical stuff like brakes.

How can I get the cable off the caliper?

my Mk1 did the same wheel got really hot and would drag, no amount of messing with the handbrake could sort it.
It was corroded slider pins. Took them out, spun them in a drill with some emery paper to remove the rust, re-greased and reinstalled. Problem solved.

That's not the handbrake adjusment, that just presets the pads after a pad change and is to do with the calliper auto-adjustment (i.e. adjusting for pad & disc wear).