I think I've cured the little truck's overheating issue. I didn't want to swap out the water pump if I didn't have to, so phase 1 on Sunday was flushing the cooling system and testing the thermostat, I'd only installed it a month or two ago, but as the overheating started around the same time I thought I'd better make sure that it was functioning, so I dipped it in boiling water and it opened fine. I flushed the system with the thermostat out and heater on, then flushed the block again to make sure the chemical flush was all gone, filled it up with fresh coolant, burped all the air out, and took it for a drive. Sadly it made little difference, the temp gauge kept climbing past it's usual halfway mark to almost boiling, so I crept home and parked it.
Yesterday, between mowing, weed whacking, sweeping, hedge trimming, cobweb removing, junk disposing/hiding, I swapped out the radiator, which was my last hope before I would have had to replace the water pump. Really easy job, so I figured that if it didn't cure the issue, it wouldn't be too painful to pull it back out again if I had to replace the pump. I had the old one out and the new one installed, in 30 minutes. I'd drained the rad into a the original coolant bottle and it was still nice and clean so I just tipped it back in, and had to add around another litre to fill the new rad back up.
Once again, I burped the bubbles out and let it run for 15 minutes or so at idle in my driveway, then took it for a 30 minute drive in lunchtime traffic (busy) in 82 deg F temps, and the gauge didn't move past the normal halfway mark, woohoo! Hopefully it's fixed, there were a couple of motorcycle "bargains" out in the country I could have bought recently, but didn't have any confidence that my little truck would have made it, so didn't follow up on them, but hopefully it's all good to go now, and I can use it. I checked the oil for the first time since I installed the new head a few months back, and I've used a litre of oil in a little over 1500 miles which isn't great, but not too terrible.