Oooooh sucks teeth This is an area of proper gardening controversy. I have seen people come almost to blows over it!
You will find some who swear by an autumn mulch, arguing that the winter cold helps to break it down and incorporate it into the soil ready for planting.
Others swear by a spring mulch, arguing with equal vigour that the nutrients are then more available during the growing season.
Personally, I think it depends a bit what you're growing and what you're mulching with. Manure is really, really rich. If you're growing flowers, it can even be a bit too rich, encouraging lots of leafy growth and not many blooms. For flower gardening, I'd say keep it to a light mulch if you're doing it now, but go heavier in the autumn. For veg, where you'll be planting in March/April, I don't think it's too late to apply well-rotted now - and to shovel it on plentifully too.
However, if your garden hasn't had much TLC for a bit, I don't think all this matters too much. The soil will need enriching with whatever you have. So I'd get shovelling if I were you!! 
If you have heavy clay, then investing in some horticultural grit and some gypsum (often marketed as the romantically-named Claybreaker) can be really helpful.