For programming and general diagnostics its advisable to have a battery support unit. I think SDD recommends more than 25A although the official JLR bat support unit is 25A from JLR Equipment.

I have an 80a Wayne Kerr bench power supply (acquired from a skip, with a blown fuse). Is there any reason I can't use this in line with the battery. I've run plenty of cars from it, but never with the battery at the same time. As I see it, it's just another battery so if its connected in parallel at the same voltage then there is no problem, but I don't want to kill the cars battery.

Any thoughts?

Don't see a problem providing the power supply is providing a nice smooth 12v dc. I use a marine 4 stage battery charger on my boat which effectively does the same thing when using 12v power when connected to a 240v landline (although being a charger it puts out more than 12v). i.e. it takes most of the load and uses the leisure batteries as a big smoothing circuit. FL2 Metropolis, Loire Blue, ebony/ivory

Cant you just connect up to a slave battery using a set of jump leads? Past: FL2 TD4 HSE Auto
Evoque SD4 Dynamic Lux Auto
Present: Audi A3 S Line.