Thank you for all your replies...sorry I haven't come back until now as I've had my two GC (11 and 7) all day and have been fully occupied!
I do have a compost bin and I don't add anything to it that carries disease (like blackspot or blight) so I might have a go at using some of that for a home-made growing medium. Unfortunately Casdon our garden soil is also heavy clay (Hertfordshire) and full of stones so I wonder whether a few bags of bought in topsoil would do the trick, mixed with the compost and then a few bags of peat free with added chicken manure pellets? Obviously I would need a different mix for seed sowing as less nutrients are needed for that.
Jaxjacky I also love Jack's Magic and have always used it in the past, but all our local suppliers didn't seem to have it this year. Perhaps ordering online is the way to go...but are the delivery costs prohibitive?
The seaweed enhanced option sounds interesting, but I seem to remember that there have been questions in the past as to whether commercial producers of seaweed fertiliser always use sustainable sources. I can't read the small labels on the front of your photo Siope but I'm assuming they state that the seaweed is from a sustainable source. If so, that's another option to research.
I'll also investigate the local council website for compost Auntieflo but knowing our council's record for providing services and helpful initiatives when they don't absolutely have to by law I suspect that may be a No! Still, yet another avenue to pursue.
It feels like a large chunk of my precious time is going to be taken up looking into this and experimenting with different options, but then gardening is one of my main hobbies and loves, so it will be interesting and worth the effort.
Again, thanks to all who have responded and Happy Gardening!