In the past twelve months my wife and I(both ethnic Brits) have

Arrived in Thailand on 29th Dec and left on 11th Jan

Ariived in Thailand on 9th Feb and left on 13th Feb

Now we're planning to arrive again in Thailand on 24th August, go off on 16th Sept (for a few days only) and

Return to Thailand on 21st Sept, leaving again on 11th October.

In view of all this, are we likely to have any problems with Immigration, and if so what should we do about it?

You are not likely to have any problem from Immigration but airline travel without visa could be questioned if you do not have outbound ticket during the 30 days allowed for visa exempt entry. If you have a tourist visa for first entry that would not be a factor.