Hi Crispy and welcome to GoY. Multipurpose compost is not going to help break up heavy clay. You need lots of good organic material such as garden compost, spent mushroom compost, well rotted manure and 6mm grit, dig in and keep digging more in!

This sounds horrible. You can either dig out the clay about 2 feet down, then truck in new topsoil. The compost must be worked into the clay. You need to keep adding compost & organic matter to re-condition the clay. This is a long arduous process.

If this oveerwhelms you its possible to do it a bit at a time if you can afford to wait. Just try improving a small area with the amount or humus-making material you have. If you spread it on top the worms will gradually pull it down for you. You want the humus material to be incorporated into the soil where the roots are, not to stand on top where it will just look pretty until it gets down under the surface.
Depending on the site of your flower bed you might like our idea - we heaped our grass cuttings on a particularly poor patch and left it over the winter. By spring there was not much left and that was easy to dig in. It made a real difference and cost nothing.
Try planting things which like heavy clay eg Solidago (the dwarf one!) Aquilegia, Crocosmia, Heliopsis,
Aster dumosus, roses, for starters. Don't be too tidy - bits of old plants will all help the soil structure as long as they aren't diseased.
Loss of any nutrients due to rotting fresh material can easily corrected by adding some nitrogen - Growmore will do fine.

Massive quantities of organic matter are needed to improve clay soil: a minimum of 20 liters for every sq. meter. That can simply be laid on, and the earthworms allowed to work, or it can be dug in 30 cm deep, for quicker action, though resulting in a less natural soil profile.

Thank you so much for the soil solutions
The recommendations are very much appreciated.

I agree with Stera, we moved here and on to solid clay, the first new garden bed needed a pickaxe - no kidding! I dump all my grass cuttings in a heap at the back of the bed, it goes slimy and revolting but eventually disappears and helps the soil. We had some heaps of wood chip from cutting down trees which were supposed to be carted down the field, but due to laziness and dispute with the workmen it got left in heaps, now the soil where they were is wonderful. My side flower bed is just dust in the dry weather (think the previous owners had topsoil put on that bed) so now it's going to get a dressing off my compost heap and more wood chippings to mulch it. It all takes time but don't get disheartened.