The charge for a filling is �42.40 and I think you will be required to pay all of this. However, if you'd had a check up and paid the �15.50 but your dentist said 'you need a filling', you would then have only been charged the �42.40 minus the �15.50 you'd already paid. A little complicated but the cost of treatment is meant to cover the course of treatment. Anything unexpected costs the full fee again.
See this website:
http://www.nhs.uk/England/Dentists/DentalCharg es.cmsx
I went to my dentist with a toothache. I couldn't tell him exactly where the pain was just a 'thereabouts' and I 'think' it's this one. Had an xray and a prescription for anti-biotics (even though no obvious problem could be found) and paid �15.50. Went back two days later, in agony, and definitely knowing exactly which tooth was playing up. Had it extracted and was charged �42.40. When I questioned this is was 'apparently' because I hadn't pinpointed the correct tooth in the first instance! Beware!