In principle you are far better quoting "on site" as you will get a higher "yes" rate for you to do the job. Someone though has to pay for the time and costs of you going to quote - maybe not that customer directly, but indirectly everyone pays a bit. So you have to be sure you are going to get most jobs you go to.
However, I am not sure potential customers realise what sort of price is the going rate for mattress cleaning, and the their expectation may be higher then can generally be delivered.
I find not all stains come out, but certainly the mattress is clean.
If I was to be doing 2 doubles at a time I would be looking for £80 ish, dont forget you do both sides.