I’m having to replace the lower tailgate straps as one has broken, but the ends of the straps that join the bodywork, and tailgate are very badly corroded. The bodywork, and tailgate look to be in good order with no rust showing.
I didn’t realise they were so badly corroded as they have a rubber boot covering them.
Any suggestions as to how to remove without causing any damage to the bodywork?

I had the same issue. heat and an air wrench was the answer

I used a dremmel on mine Previous cars:
S-Max 2007-2013 (only diesel I've had, good car but expensive when diesely bits go wrong, so what's the point?)
Galaxy 2001-2007
Mondeo Estate 1997-2001
Sierra Estate 1993-1997
Uno Turbo 1987 -1993
Fiesta 1984 - 1987
Fiat 127 1982 - 1984

An old thread, but I've just had the same issue so thought I'd propose another answer. Unfortunately, I didn't take any photos.

This job creates a lot of rust flakes and debris, so first job was protect the boot lid carpet. I put a cloth behind the strap to protect the bodywork and held it in place with a little masking tape. This also collected the rust as it fell.

I started by spraying all the bolts with WD40.

Top ones: My straps were rusted to the bolt, so didn't rotate around it. This enabled me to use the strap to turn the bolt. The right hand one I managed simply by pulling the strap upwards. left hand, I hit reasonably gently with a hammer, using a screwdriver as a chisel on the strap end, to turn the bolt. I then had to dremmel off the end of both straps as it wouldn't pass the body work. On the left, that results in the dremmel kicking flakes of metal up in the air, so wear some eye protection! That done, I used mole grips to remove the bolt.

Bottom ones: These weren't so bad and I was able to undo these with a torx head and a socket set.

Just came on here to see what people were doing in the way of preventative maintenance. It seems a shedload of grease is the answer.