Chill guys!!!!!
I decided NOT to put any chemical crud on the frames incase of damage etc etc.
When i cleaned this particular house again i gave one sill a really really good scrub with plenty of elbow grease and a wet clean cloth and it came up quality, but it took ages someyhing like 7 or 8 mins, but i made it an extra special go, to show customer.
I have now left the ball in their court and said i will do all frames sills etc etc but will charge an hourly rate to complete job as it will take a while to do.
They are going to let me know next time i clean if they are going to go for it, they were happy enough about paying for it this way but i think she is going to get hubby to have a go to see if it really is worth it.
i bet they get me to do it, or should i say he'll want me to do it, a proper arm ache he will have

Andy