Hi Will,
My first car was and is my Herald 13/60, though I’ve now had it about 14 years! I also had head gasket problems, about 3 months in. Though they only became apparent when it shot into the red on the A1 just north of the M25 in early December, that wasn’t fun… Anyway, that HG change was my first major job on the car and it was pretty straightforward for a classic car novice, so you should have no trouble!