It's a bit of a grey area but technically sub contractors are supposed to be using all their own equipment, van etc, if not, then they are your employee. A guy who cleans windows in the same area as me was telling me about how he was fined off hrmc for using his dad as a sub contractor in similar circumstances.
Yes and in addition to this i also was stung by HMRC for a simular error.
Best thing to do is make sure that they arnt just window cleaning for you. If they are sub contractors they should be able to prove that you are not thier only customer.
For example if a window cleaner worked for me on a sub contract part time basis, id expect them to have thier own direct customers that i can take references from as proof of sub contractor status. (They may sub for whoever needs window cleaners wherever)
If they dont have any other customers, then you are employing them and HMRC will expect you to know this.