Good advice. After driving around for a week with half the dash missing while trying to trace this fault on my 1989 245, I removed the turn signal fuse and found that the fuse holder terminals were caked in corrosion. After a good clean of the terminals and fuse, I popped it back in and hey presto the turn signals worked again. In fact, I removed all the fuses cleaned (or replaced) them and the terminals, so hopefully that should head off any more electrical problems for a while.
What I discovered during the course of having no indicators was that the relay controls the hazard lights and the seat belt warning light. The hazards are on a different fuse and supply, so worked fine while the indicators and seat belt warning light were out of commission.
Also, found out while rooting around under the left hand side of the dash that the single inline blade fuse close to the fuse box is for the alarm which arms itself when you remove the fuse and then replace it :-/