I have now resolved the first problem. I was trying to enter data correctly into the subform on the main form and I also tried it on the separate form for payments. The thing I did wrong, in my opinion, is that I tried to put the subform onto ClientF instead of the the tour detail form (ClientXTourT) and there was no proper link between the payment subform and the main form.

When I removed the subform from the Client form and set up a new subform on the Tour Detail form, which was based on the joint table ClientXTourT, everything worked perfectly.

Could I have created a Query, based on the three tables, ClientT, TourT and ClientXTourT, and still kept the subform on the ClientF form or is that just too complicated?

Your solution to my second problem, using a Composite Key for the two fields, worked perfectly thank you.

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Reply from Alex Hedley:

You'd need the ClientXTourT as a subform to keep it contained.

Glad 2 worked :)