In your letter did you mention that you would like the company to absorb the extra payment as a gesture of good will as you are a loyal returning customer and you want to retain them for future repairs. This is known as the soft approach,the hard approach is usually dismissed immediately.
To my mind I wasn't soft or hard with my email. It just seems that they got the quote wrong & somehow misjudged the work involved, so want me to pay what they should have quoted in the first place.
I think I worded this thread badly from the start.
All I'm really trying to figure out is a possible reason why they had to unexpectedly drain the tank for an extra £55 and they couldn't have included that in the quote from the start?