Booked her in for a full underbody/engine steam clean to get rid of the general grime in the engine bay and the years of waxoyl on the underside. It seems that rather than just 'top-up' the waxoyl, the previous owner just completely resprayed over the top, religiously, every year.
Yesterday I decided to drop the fuel tank to fix a leaky sender gasket. I had to grind off all the bolts securing the tow-ball assembly to the cross-member and chassis, as were so rusty as to be now single pieces. Junked the whole assembly and support rods as I'm not gonna tow anything. Looking at the tank, I can't even see the sender and the bolts that hold it to the vehicle, as there is so much gunk covering everything. Hence the steam clean appointment.
Once it's cleaned, I'll look at the tank again.