You have been helpful though...always a worry when these things happen....due to pick up a 'spare' car for winter next week, so not the end of the world...thanks for your help.
Possibly either the two croc clips touched together (shorting out both batteries) or you connected them the wrong way round (giving you a 24V 1000A short circuit, very briefly). If so it may be that you have damaged the wiring to the battery, it would be sensible to check it over.
You have been helpful though...always a worry when these things happen....due to pick up a 'spare' car for winter next week, so not the end of the world...thanks for your help.