I have spent quite a time searching for discreet looking indicators and have just fitted some excellent units from Staffordshire Vehicle Components (01827 67714). Small enough to be unobtrusive but very visible when lit - and not at all out of place on a period vehicle. £16 a pair and come with a handy fitting bracket. See here:

http://www.s-v-c.co.uk/prod/svc-indicator.html

A tip - I wired up the front units first and wanted to test the flasher relay. The lights lit up but no flash... All the power was going to the right places and the relay would 'click' when directly earthed but refused to operate when wired normally. Puzzled, I trawled the internet and found a mention that the relay will not operate unless there is a sufficient current draw. Wired in the back units as well and the relay sprang into life!

.

Thus you will know when you blow a bulb!!!


Mike

Nick - I have also fitted those (pictures here :
http://pub25.bravenet.com/forum/2099944454/show/641538 - scroll down to my posts) - only bought them at a different source.
I used the mounting brackets that come with them in such a way, that I did not even had to drill any new hole in some part of the car - this is the reason why they are mounted on top of the taillights.

Regards,
Ralf