You do not need a CIS certificate as a window cleaner, window cleaning is not classified as part of the building industry.
check with the tax man, he will tell you the same.
you can get one if you like, if you do a fair amount of work for builders then you will get a healthy rebate when you submit your tax return!
If you only do a job here and a job there then don't bother, but it is a good way of covering your tax bill.
Ian