Darren
Don't go in too cheap.
They won't always go with the cheapest. When I started out I picked up two residential homes, one in Dromore the other in Banbridge ( I'm in Belfast) I priced too low, got the work, hated doing it and was glad when the contract ran out after two years.
You mentioned the job is 25miles from where you live, that is aroung 1 1/2 hours downtime every time you go as well.
Price accordingly
Also, You didn't mention the frequency of clean which will affect how long it takes
Sounds like a long half day to do this job, so add up what you want for that then add on an extra 10% for unknown factors eg, access, getting the job signed off, and if you are on a 12month contract inflation.
Hope it goes well
Ian