'Clay' covers a multitude of sins. Ideally you would know how expansive it was, which depends on how pure the clay is and what type of clay it is. I expect there are articles on the intertubes explaining how to estimate these if you don't have a soil survey to hand.

It depends a bit on what your requirements are for the office. Conventional wisdom says you need to remove all the topsoil (is that the 300 mm?) before anything else. A raft will certainly work, as long as you don't mind your building moving around a bit if there is any heave, but as you say, it may be a bit OTT.

All I did for my garden shed was to take the surface off to make a level surface, then put a layer of sand as blinding and some paving slabs I happened to have as a 'foundation'. Then some fence posts laid horizontally and then the floor of the shed. The floor is a bit wobbly anyway since it's a cheap shed but if the ground does move (it's right under an oak) then I expect I'll need to bash a few wedges in to even things up.

I presume a SIP kit is a bit more expensive so you might want to take it more seriously. What is the floor stucture - how is it supported?

I've no idea what kind of clay it is unfortunately. Yes 300mm of top soil then clay.

The floor structure is a SIP so they are 150mm thick and I am pretty sure I need to keep them dry as well.

I'm spending about £4k on the building + foundations so I'd prefer something that won't be err weak and wobbly.

I've found a couple of different methods which sound like they might do the trick:

ProBase sounds a bit like a fancy version of @djh paving stones: http://www.shedbase.com/probase/
Swift Plinth sounds like a bit more work but also more forgiving (you can adjust it later) https://swiftfoundations.co.uk/swift-plinth-plus/

I really don't want to do a big concrete raft!

How about a local steel fabricator to make you something like this:

https://www.google.co.uk/search?q=steel+conservatory+base&rlz=1C1AFAB_enGB461GB462&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiEvv2xn7rUAhWPZVAKHaDRDcgQ_AUICygC&biw=1350&bih=643#imgrc=Y4tKkZfOKIyXWM:

Will the floor span from edge to edge? i.e. could be installed clear of the (exposed) clay surface? then all you need is suitably supported edge/ground beams. How about a farmer sinking some gate posts (with his tractor mounted auger) as piles? Railway sleepers (or longer/heavier 'crossing sleepers') as ground beams.

Google "conservatory foundations screwpile" and see if any of the systems will work for you.

Thanks for all your help. I had a look into the soil and the best ways of mitigating the clay. Seemed like compacted type 1 with the quickbase was going to work - but a lot of work!

I had also looked into screw foundations but the only company to give me a quote was £450/sqm - that would have made this project very expensive!

I gave up.. but then found a company that will do them for £37 per screw - much better!

I'll have them around in a couple of weeks.

Thanks again for your help

Do the sip company recommend anything that you have to comply with to get the warranty?

For my timber clad outbuilding (on clay) I dug out about 150mm and compacted the ground with a wacker plate. Then put down a layer of hardcore then a concrete slab with the top above ground level. This was made fractionally under size so any rain dripping off the cladding lands on gravel around the slab rather than the top of the slab. I put two courses of engineering bricks and a DPC on top of the slab so no possibility of the sole plate getting wet by water blown under the cladding. Overkill perhaps but It's a pain to fix a leaks if the design is the problem.