Kay has suggested everything i was going to suggest Roger, in particular the cracked liner which seems quite common on white blocks.
If you have a boroscope/endoscope, get each cylinder in turn to the bottom of its stroke and have a good peep around inside the liner through the plughole. Closest i get to gambling is a few quid on the lottery now and again and once in a blue moon a quid on a sweepstake somewhere (work, pub similar, not through a bookies or similar) but i'd be willing to bet the 5p piece i found earlier today that it's either a cracked liner or something else.
The something else could be HGF again - or was it HGF in the first place? Could have been the cracked liner then, but only when it got hot, perhaps after a long journey towing a caravan and then stopped and the heat climbed (opening the crack) through lack of circulation of coolant until it cooled to a more normal level.
Could be that while giving all the symptoms of HGF that either it was only the cracked liner then, or possibly because of compromised cooling the head gasket failed as well, giving a bit of a red herring. The extra heat from the head gasket could have expanded the crack in the liner or even just use.
The gearbox relies on feedback from the ECU if it's electronically controlled, if the ECU tells it the engine isn't performing then the gearbox will "go easy" on things.
If all else fails, drop a 1UZ-FE and A341E into it instead - that should drag any caravan nicely and give you an extra 50bhp.
Alternatively, drop a B230FT and AW71LE (aka A341E
) in it with a higher pressure boost and drag any caravan you like.
Whichever way you look at it, it sounds like a hefty bill just for parts, depending what you find with the boroscope.