Based on what you said about 2 guys 4 hours I would charge approx £400-£450 on a regular day 3 monthly schedule. That's 4 hours hard work and the amount of water it would use would be loads. Plus as you said longer poles etc needed which all cost money. If it was irregular I'd add say 25%.
We've got a 6 weekly job at £390 takes two of us 3 hours, so that's how I based my pricing.
I reckon the commercial cleaning company is doing it for no more than £200.
Commercials always seem to be quite low priced I have found.