Refer a friend letter

Hi,
I currently have 90 members on my monthly service plan (Boiler/plumbing), which is a bit like 'British gas homecare' but with more of a local company feel.

I know all my customers personaly and have a great relationship with them.

I have been advised to apeal to them to recomend a friend or family member for a small Incentive (£50 marks & spencers vouchers) to join the service plan.

So I am asking whether anybody out there could come up with a great letter promoting my offer.

I look forward to hearing from you!