Recently we've been having a problem with the Kombi (75 2L bog stock). It will start fine, but when it's hot, say after a 15min drive, and you stop and then try and restart, the starter motor struggles and sounds like the battery is flat. If you leave it for around 30min and try again, it starts no problems.

To me this indicates a temperature related problem, but what is causing it. Any ideas?

I would get the starter motor looked at. It may need overhauling.

Actually thinking for a second, heat creats resistance, so it may also be earth strap from gearbox to body, or main power lead to starter also. It will be a main power or earth lead.

Starter motors do have problems when they are worn and they get hot too.

Yep as Hell bus says.
Check your leads first for breakages and bad connections at each end as their easy to do yourself.
Also be sure to check the power lead from the ignition switch to the relay on the starter, they are prone to suffer voltage drop.

If this does not help it's time to look at the starter itself (assuming the battery/charging is fine).

starter motor solinoid perhaps?

Sounds to me like a combination of two things:

1. The carb gets hot when you've been switched off for only 5 minutes, and so the fuel vapourises which stops it flowing... and you have to wait half an hour until the carb cools down again. I've heard of several ways people have attenmpted to fix this, everything from making aluminium heat-shields for the carb to wrapping the carb in a wet rag. I've also heard that a bad coil can mimick a vapour problem.

2. You say that "the starter motor struggles and sounds like the battery is flat"... I assume this means the starter turns the engine over in a slow & sluggish way? If so it could be caused by an old tired battery, and/or a bad connection between starter and battery (either the fat red wire, or any of the ground straps). But this is definately NOT a sypmtom of a bad solenoid: if the solenoid is working then the starter speed is not affected, if the solenoid is not working then you get NOTHING from the starter. A solenoid with dirty contacts can intermittently prevent the starter from turning, but again you get all-or-nothing.

am following this thread as had a similar problem a couple of times ,where the motor turns over very sluggish but wont start and mine is a efi caravelle and the battery is only 8 months old

happened again today ,was at home so investigated some things, charging at 13.5 volts ,starter felt very hot ,even tried jumper lead from batt to starter ,to get it started, jumped it from our holden,ran it a couple of mins then all was well,still very puzzling,

plus also the quality of fuel that you are using may contribute to the problem as well..
a suggestion if you are not already doing so..
use a higher octane fuel and run some flash lube with it..
cheers paul