is £100 to steep

hi
i put my car into garage to have a head gasket change as it blew
garage quoted me £450 so i said go ahead with it it had been with them a week so i asked when they were likly to start the work they said the following monday 3 days time
i seen another car to buy and when i phoned them up sunday night to stop the work they said to late as the mechanic has already started on saturday night so he could have a gasket ready for monday morning and i could get my car back fixed

i said i still dont want the work completed and how much to have it put back together as it was they said £160 so i asked how much to just take it as it was he said £75 as the mechanic has already put 3 hours into the job and they were charging £25 per hour i said ok ill do that as ill send a transporter round to pick it up as a friend has one

come monday they told me they have had to charge another £25 for a mechanic as he had to do more work on the car as the engine was suspended by the trolley jack that he had to put the engine mounting on to secure the engine for removal

so now they want £100 for not completing a job or not fixing my car

ok i know its not there fault the job terminated halway through but what is a fair price to pay is £100 to steep is my question?