Poor starting after a long run

[dg200_glider_pilot]

My car starts well first thing, with a good turn of the starter and quick leap into life from the engine - my assumption is that the battery and starter are OK. However, after a long run of more than an hour at normal speeds 50-60 ish, after stopping for a break, the car is very slow turning over the starter motor. If I put the booster pack on, or roll it down a hill a bit, it starts first time. I assume that the battery isn't getting charged correctly.
What do I check and how do I fix it?
I have a volt meter - what should I check on the voltage regulator?

Matt - Minor 1000 1098 Convertible 1965 Yellow - Dynamo Positive Earth.

[gairlochrosie]

Could be a bad earth / poor electrical connection and or the starter motor requiring serviced.

[chickenjohn]

Could also be a loose fan belt. If the dynamo is not being spun efficiently it won't charge the battery very well, either that or the battery is on the way out (especially if the car is not regularly used and the battery is 3 or 4 years old).

[Willie]

Also check the voltage across the battery when you have stopped after a run when it should read just above 13volts.

[overider]

A quick first test is to drive up to something like garage door put the lights on and rev the engine if the lights go slightly brighter and then fad back as the revs drop off the charging circuit is ok.

[kennatt]

ive had this a few times on various cars,each time it was the starter motor,when the heat from the engine transfers to the starter,it causes an electical breakdown and the starter to spin slowly. But no problem when cold.Don't know the true electrical reason not that knowledgable on electrics. Can you borrow a known good starter to try before you buy so to speak. Cheers