Getting a refund for poor service

Hi

I have a complaint against a company due to poor poor service.

My first letter has returned the initial amount I paid for the service + a voucher to redeem against future use with the company.

However I'm not satisified with this as

1) Their incompetence meant I had to incure extra costs. (which I had detailed in the letter)
2) I will never use their service again, so the voucher is no use.

There are more grumbles I have with them, but these are the main two.

This is a National company with the resources to do more than they have offered. However, the amount in question is not more than £50

I intend to follow it up. I know I need to send back their cheque and the voucher with a further letter, but it concerns me that this may be the only time I will see my money returned.

Do any of you have experience with following these things through?

Any tips? Do you think they'll return the cheque (with additional money) to sort it out?

Any help gratefully received.