[mdjlucan]

What's the best way to break the clay soil down on my allotment

[mumofstig]

Dig/fork it over while it's damp, once it's dry it's like concrete - add as much compost or manure as you can get hold of.

[grinling]

Used to get manure from stable in Nazeing. It was also a dog groomer. Have a look on preloved

[Plot 1 Problems]

For me it was a three pronged attack:
1) Fork it over in the winter when it's not water logged to break it up.
2) Cover it in compost/ manure
3) Get green manures growing in it whenever you are not growing crops to prevent the rain from compacting it or the sun from baking it hard. Always have something growing in the soil, even if it's weeds.

[al78]

If it is compacted then brute force with a fork, then spread organic matter over it, and spread as much as you can get hold of every year. The soil will improve over time, but don't expect instant results. My allotment was very cloddy with some bits very compacted when I took it on seven years ago, but now, even with the bone dry weather of the last two months, I can fork perennial weeds out fairly easily much of the time. Unfortunately SE England seems to have borrowed a semi-arid climate from somewhere, so you might need the assistance of someone strong to dig it if it is like concrete, and buy a decent fork (I've bent prongs and snapped a few digging very compacted clay soil over the years), I recommend one that isn't made of wood, the metal ones don't snap.

[Goosegirl]

Apart from what has been said, when you're digging your trench incorporate some grit into the bottom then mix some more into your back-soil.

[Hampshire Hog]

You can also check out adding some chemicals to improve the soil structure.
Have a look at the RHS advice on clay soils they mention use of lime or gypsum depending on the PH of your soil. They also have useful advice on what to do/not do when cultivating on clay.

[al78]

Some advice from the RHS here:

https://www.rhs.org.uk/advice/profile?pid=620

According to them, it's not worth trying to incorporate grit except on a very small scale. The amount you would need to make a significant difference on the scale of an allotment is impractical. Just add organic matter whenever you can. Also, avoid walking on the soil, especially if it is wet, make your beds narrow enough that you can reach the middle from paths on each side.