Right after discussions with two local camping shops I ended up getting 2.5 litres of Fabsil. Had to paint it on myself as would have needed too many aerosols to do the same.
What seems to be neglected in these discussions is that my family tent, when erected as advised, is flipping difficult to paint! Took me ages to do it. I really hope this is not an annual thing!
To reach the top I attached the paint brush to a garden hoe. The brush fitted snugly on the metal fixture. Mostly worked well in the midday sun until I ripped a 130mm hole in the “canvas”. Off to the camping shop again for Kampa tear repair. £5 later I discover that, despite advice, the tent needs to be disassembled before the patch can be properly applied yet the Fabsil needs at least 6 hours to dry.
I won’t mention the hassle it took to get into my folks reasonable sized garden to have space for this in the first place as they were on holiday…