If it is for spuds you have several options.
1) Spread the manure and leave it on the top until you are ready to plant then turn it in.
2) Spread it and dig it in now.
3) When you dig your trench for sowing the seed potatoes, place a good thick bed of manure in the bottom and trample it down, place the potato on this, layer with a little more manure on top of the potato before earthing up.
If time is not an issue - option one now then option three when you sow. If time is of a constraint, Personally I would go for the third option. This will allow you to continue work on other areas of your allotment and will still give you a good crop of spuds.