New plot is shallow.

That sounds a bit ominous Ben, and clearly, you're hitting on the right solution! A good heap up will make all the difference

Just to make sure, it's not a hard pan of subsoil is it, or is it spoil? Presumably you're getting good drainage though.

Give my best wishes to the home of the 'School of Mines'...;0)

It might feel like big rocks but it may be  ahrad pan of compacted soil with gravel ---- that is what was under my plot about ten to tewleve inches down.

If you can wiggle fork times in and start to break it up it will help a bit with drainage.

Big rocks of course could be dug out. We had masses of staones of all sizes and when lots were taken out our previously raised beds became sunken beds (much to everyone's amusement) but by adding compost, spent grow bags, small amount of manure - all we had- the soil is bulking up nicely now.

Having raised beds means we are improving growing areas only and not the paths

I'd ask fellow plot-holders (if any) for their advice on the ground conditions and also dig at least one test hole at least 2' deep (using a pickaxe if necessary) to find out exactly what I was up against before planning my next move (the 1st option is much easier!)

Thanks all for your responses.  There's definately big rocks / boulders under there as I dug 2 out when I hit them which wasn't easy, and I'd say they were about half the size of a football.  They're rounded / smooth edged as well and don't have jagged edges at all.  Maybe it's as our site is alongside the River Tafff, but that doesn't explain why this plot has the problem and our other plot, although stdoes not when this one is further up from the banking.

I shall take the advice of JayG first and seek the advice of some fellow plot holders.

when this one is further up from the banking.

Is the land sloping? If your new plot is at the top of the slope, and, knowing how much it rains in Pontypridd, it would probably have lost some of its topsoil to those lower down.

You have my sympathies.  Our site was originally a field which was then covered with the debris from the WWII bombing campaigns, had some topsoil put over it and turned into allotments.  My recommendation is to gradually increase the depth as you are doing with manure and other compost gradually developing raised beds.  I used the lasagne gardening method to increase the height of my beds - begging grass clippings, garden prunings and cardboard through freecycle and using manure from local stables.

We have the added complication that the field was used to grow onions in for 20 years and so we also have onion white rot in the soil!