So I had a look last night. Turns out the problem exhibits itself on some occasions when he receives a reply to an email of his. The 'original' text shown in the reply is missing the odd character, and (I think) some of his responders point it out. He does a lot of work with English proof readers and editors (human beings variety!) so I guess they are likely to spot such things.
He couldn't find any examples at that time (that were a number of other issues to sort) so I've asked him that when it next happens he finds his original email in the Sent Items mailbox and compares them.
He runs NOD32 anti-virus which checks his outgoing emails and 'signs' them. It's noticeable that the signature is not on the Sent Items copy so is presumably only being added as it actually goes out. Could NOD32 in some way be the culprit?
All the keyboard settings seemed fine and, although all the typing I did whilst there didn't show any apparent problem, I left a spare keyboard 'just in case'.
Anyway, I'll wait for more info.