How about this,
Get some vouchers printed up for "Free Stain Removal" normally worth £49 or £35 or watever you would charge.
You can then give these as rewards to customers who reffer you.
That way you kill 2 birds with 1 stone
You get a new customer
and you get back in the house of a old one
Most customers wont let you clean for free anyway they will feel obliged to give you something, or will at least ask you to clean something else while you are there, so its unlikely you will leave empty handed.
You can also give these to customers who get there carpet protected, as it will give a larger percieved value for the protection so should make it easier to sell.
At the quote or once you have finished cleaning, you offer them the protection at say 50% of the cleaning cost, and tell them that with this protection they also get a free bottle of stain remover and a voucher that entitles them to a free "emergency stain removal" should they find something they cannot remove. Obviously it would be a idea to have a cut of date on the vouchers if your gonna use them like this.
And heres a link to some
http://www.viking-direct.co.uk/catalog/catalogSku.do;jsessionid=0000hPIiT5hcAm0JR0TNfkfk1_h:11g06dj48?N=2&pr=Q2H&id=44246