Thanks for your help, it fixed the problem; but this is strange my Notes client (8.5.2) has a 8.5.1 (Domino version) design, and seems to be locked on that design version whatever I do.
On [most] Domino servers, the “Design Task” runs automatically at 1:00 am every day. When this runs, it refreshes the design of every mail (or other) database on the server, using the template that is on the server. If you want your mail database to retain a particular design (in your case, the design of the mail template on your client. select File/Application/Replace design and un-check “Inherit future design changes”. This will prevent the automatic server refresh mentioned above.