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Thanks for that ...

Its very critical that you remove ALL debris and corrosion from the grove that holds the seal. Use Red Rubber grease to assemble the seal and piston.

AND!! If you use air to pop out the piston???
Don't keep your finger in the way holding the caliper body! Or else!
It will leave a mark! And your finger will be sore for a few days!
You can guess why!

And, as others have typed. put the seal around the piston first and hook it into the small metal grove at the caliper bore.
Then slip piston in working the seal around. I use a popsicle stick to slip around and work it.
Good luck, Patience works well!

Thanks for all your advice guys..

Managed to get the piston out with, wait for it, a bicycle pump with adapter wedged into banjo bolt hole, and lots of oil and a heat gun . As you can guess, I dont have a compressor!! I'll try and send a picture, do you think its serviceable ??

What do you think

The bore should clean up fine. What's harder to discern is the condition of the piston. Clean off the grunge and look for pitting. Pitts = trash can fodder.

There are a few tiny, tiny pits . I've polished and polished it. It's as smooth as a babys bum to the touch but the pits are still there. Still trash ??

This is the worst bir. Dont suppose the piston would ever come out that far ?? I know you shouldn't save money when it comes to brakes, but, £120 for 3 piston!!

Did you have the dimensions for your front/rear Pistons?

Take measurements of your pistons, diameter, height, location and dimensions of the seal groove, then ask board members here or at a GS site to measure pistons from a similar year GS 750/850/1000.
I say this because Kawasaki, Suzuki, Yamaha and Honda often bought sub assemblies from the same vendors. Sometimes identical, and sometimes with minor changes. If these pistons are interchangeable, you can get new oem Suzuki piston and both seals as a set from Partzilla for $25.
I ran into this when I recently rebuilt the front calipers on my ZRX. They are the same calipers as used on the Hyabusa, except Suzuki specified light weight aluminum pistons instead of steel, and Suzuki sells the pistons and seals at a fraction of what Kawasaki wants. Identical dimensions, lighter weight, much cheaper. Win.