In the corner behind the glove box near the joint between front and rear looms IIRC. Not sure if present on early 1600?
Nick, I can't thank you enough!
After another 90 minutes of 'contortionism', I've just found it. What on earth possessed Triumph to make a fuse so inaccessible!
I'd almost given up hope of ever finding the fuse when I spotted a red flex which had apparently been rubbing against something and was showing a tiny piece of bare wire. I gently pulled this down to make space enough to wrap some red tape around it and, hey presto, the fuse holder came with it!
Fuse now replaced and side/rear lights working again. Presumably the bare wire was the cause of the fuse blowing. I'm now having my well deserved cuppa before I go back and replace all the trim etc.