Is Leafmould acidic?

I think it depends on what sort of leaves - conifers/evergreens will be more acidic than deciduous trees. I think the major potential problem with leafmould is that it can have a fairly high ratio of carbon to nitrogen if it is not well rotted. Microorganisms working to break it down can then pull nitrogen from the soil, depleting the levels of this important element in the soil (this is how those little bales of straw that you put into your pond to prevent algae work - microorganisms pull nitrogen from the water which means that the algae don't have enough to carry on growing and the water clears).

But if it's done, I'd leave it. Make sure you keep fertility levels up (peas and beans will make their own nitrogen) and look forward to some beautifully crumbly soil in a year or so's time when the worms have pulled it into the soil.

Partially decomposed leaves will be fairly acidic as the production of organic acids is part of the decomposition process, but these in turn are broken down later in the process so "finished" leaf mould should be near enough neutral (this is also true even for pine needles.)

It therefore depends on how well rotted your leaf mould was, but I wouldn't worry about it anyway - it's only a temporary effect and after a winter sat on your lotty a very minor one I would have thought. 

Thank you all for your in-depth replies!
I will move some of the layer but not all of it and I will Lime in the Spring where required.