got a pre-fab concrete garage with a fairly flat wooden roof. think the previous owner has possibly had it re-felted at some point with the installer using some longer nails than necessary in locations that have come all the way through and are now causing leaks inside. Thinking a coat or two of bitumen (or other) would be a fix that should last a few years until we come up with something more robust (or a new garage!)

Any recommendations for which bitumen paint (or other) to use to seal it?

Flexacryl – don’t bother thinking of anything else.
Its on offer at either Screwfix or Toolstation right now too.

Have you considered EPDM Rubber roofing?

Better Google rubber roof to mske sure I have the right initials but is really easy to fit and meant to last 30 years.

Nail some sealskin socks to the roof, overlapping obviously.