It makes a huge difference whether you're a Sole Trader or an employer.
For Sole Trader, try MMA Insurance plc (through any decent broker) who do a "Master Tradesman" policy which they modify to include window cleaners. This has the usual "not the property being worked on" clause, but this is intended for such things as a kitchen worktop that some dolt cuts in the wrong place, etc. From the w/c point of view it means the actual window you are in the process of cleaning, not the building/premises you are working on, so unless you are b*** clumsy jit won't make any real difference, especially as you will have to pay an excess as you would with any insurance.
Mine was about £75.00 for £1mill cover
Cheers,
Ian