Changing Test Time

[AliMak]

i booked my test for 4th of June, was the soonest they had but just went on the site and checked for earlier times and theres 1 on 14th, then went to change it and it costs £10 more. is that just because im changing it there charging more.
It will cost £58, when it cost £48.50 before, anyway i want to change it to earlier date, so dont mind that. But im going to be checking all the time for earlier dates, just wanted to know if i take that time, then find a earlier 1, will i then get charged £10 again. So be like £68 if i find another earlier 1 later on?

Any info be great
cheers