I used the ATM near the beach, in September 2012 last so it may have chanced but back then the machine gave two options, a guaranteed rate of 1.14 or the prevailing rate when the transaction goes through your bank. I took the later and when I returned home I checked my bank, the rate charged was 1.251 !!! reduced by my banks fixed fees to 1.219 on a withdrawal of 200 euros (so with a larger withdrawal the rate would have reduced by less).
Significantly better than the 1.14 guaranteed, not sure if my bank would have charged their fees on the 1.14 but I expect that they would so reducing the rate to 1.10.
Potentially £16 more expensive on my 200 euro withdrawal
If the ATM software is the same take the non guaranteed rate.