Daisychain,
My first advice would be check your contract - you are entitled to a written emoployment contract. Also get specifics from your employer as to what will be involved in your work on site - confirmation of the 3 months, any extended working hours (some site work can be pretty pressured and involve working extended hours / weekends).
I work in the construction industry - engineering / project management. It is assumed in my job that I'll be expected to go to site at some point (as it turns out, I've been site based for nay on 4 years and have never been office based - I find the experience on site invaluable!!). My contract says that I'll work pretty much where my employer wants as each project dictates - it's worded suitably fuzzy along the lines...'your work will involve travel to sites from time to time, etc etc'. I'm quite happy to move about at the moment (only within Dublin, and within reason) but I put the foot down when it comes to going to jobs outside Dublin (some of our work is in Sligo, Galway, etc). Check your contract - If your contract says you're office is based in A and they want you to work in B (site), then I think it's only fair and reasonable that you should be reimbursed either for mileage, particulalrly if your running your own car or a car parovided (my company does not provide company cars). It may even stipulate the mileage in your contract (mine does).
Quinno