MikeB are you by any chance in Bournemouth ?
The other Mike has just contacted me by phone having just read your post, he's still not able to post replies himself on the forum due to some mysterious technical issues that are as yet unresolved.....he'd like me to say that his experience of trying to access tickets of his choice was identical to yours.
His remedy to a frustrating experience was to get on the phone and speak directly to a lady in the box office who willingly sold him the seats he wanted.
Happily he is going to be on the terrace on the night of the gig.