I think its a combination of the weather and your soil are you on clay? I get this when double digging my heavy clay, when I pour water in, even in dry conditions it takes forever to soak away.

Probably an idea to look at a raised bed? You do have a serious drainage problem which plants won't like.

And you are in Norfolk! Normally driest part of the UK. I would guess that it is the water table that is very high after so much rain, but if your land is often this heavy and difficult to drain, the only real solution is to put in land drains which take the water away. It depends what you are thinking of growing too. There are many trees and shrubs which enjoy extremely damp conditions, such as willows, alders, poplars and so forth. Likewise various flowers which grow well in damp, such as Ligularia. If you want a vegetable garden, you should adopt Bulbaholic's idea of a raised bed, which will grow better produce in any conditions.

The thing I'd be worried about is precisely what is stopping you from going deeper - sometimes pebbles and stones sort of 'weld' themselves together to make a solid barrier, but the other possibility is there's concrete there - and if there is, what's it there for? I'd suggest a jackhammer, but if there is concrete and it's part of the sewerage or drains, then that isn't advisable.

Thank you very much everybody. The salad vegetables will be in a raised bed that I have yet to build. I will be careful about what I choose for this particular spot.

Sounds to me like the soil is very compacted and that is causing the water retention, so I would fork over (not spade) over the whole area and hope that helps with drainage. It could be a high water table given Norfolk is a low lying area in the main. It will certainly help plant roots if the soil is not an impenetrable barrier, even in a raised bed.

The last time I saw something like this was when I was excavating a hole at a friends house and found a solid surface. It turned out to be the road used by the builders which had been covered with soil. I'd try digging a bit further away and see what it is like there.

Thanks for the additional comments. I will go further down and see what's there ...... (oooooh scream coming!)

I can't blame you for the scream, Ann! : )
From your description of the stones, you might discover an old road there, or bedrock. If the former, it will take major effort to get through it, but there should be decent drainage underneath. If the latter, it's time for raised beds, containers, mounded rockeries, etc...etc....