So, wanting to build a reasonable shed / workshop in the garden. Very happy in any form of DIY but time is a big issue for me. What decent kits are there out there to chuck together?

I guess the nearest thing you’ll get to a ‘kit’ would be what seems to have been dubbed a ‘log cabin’.
They don’t actually feature logs, but thicker-than-usual boards that overlap at the corners:

Otherwise, sheds tend to come fairly complete with just the walls to screw together and roof to put on. I’m assuming that this isn’t what you’re after.

Alternatively you could maybe find a cutting list online for a shed of the correct proportions and send it to a decent timber yard.

We bought ours from Tuin, took a day to build plus a day for the roof.

Very pleased with it, as simple as building lego

Can’t recommend “Billy’Oh” or any similar priced online sellers. Thin crappy tat. We had to get something in a hurry for the Archery club and delivered to site, so had few options. As a serial DIY shed-builder, putting it up made my teeth itch. Horrible. Had to go to my builder’s merchant and buy some OSB to overboard the floor as I nearly put my foot through it. Ive seen more wood on a chopstick than in the floor bearers too.