For me that would also have been about £30 for an initial clean, but only on condition it was prior to it becoming a regular clean.
If it was quite literally for a one off clean I'd like to think I'd have the nerve to charge about £50....except, sigh, I know I'd end out only charging £30
A monthly clean would be the same as most the other guys who have replied, around the £12.00 mark.
My method for the one off clean would be entirely WFP, I did a clean similar to this the other day, bigger in fact and the conservatory was minging, covered in green algae and assorted detrius, entire job took 50 minutes, all done WFP and I charged £30 (was a customer that squeaky dropped some time ago).
Regular clean is going to be £18...so I guess if it was just a one off clean I'd have been underpriced on it...thought it was ok though as it is now a regular job.
Ian