I'm with Themas on this one (as usual).
I think that the relevant legislation he refers to is the Acess to Neighbouring Land Act 1992. It applies in England & Wales only.
It's scope covers this:
The maintenance, repair or renewal of any part of a building or other structure on your property,
The clearance, repair or renewal of any drain, sewer, pipe or cable,
The treatment, cutting back, felling, removal or replacement of any hedge, tree, shrub or other growing thing which already is, or is in danger of becoming, damaged, diseased, dangerous, insecurely rooted or dead
The filling in, or clearance, of any ditch.
I'm not sure whether window-cleaning 'counts' as maintenance either. You could write a letter quoting the Act and the terms above and see if that gets the wind-up him. If he still refuses, you can apply to a county court for an order for basic preservation works to be undertaken. Clearly that's where it starts costing money.