I know with my contract the hours outside of the regular working week would have been paid at the same rate.
I second this. And I think that unless overtime terms are stated explicitly in the contract, its up to the contractor to ensure that the terms are agreed/understood by the client (and agency) before working extra.
I've had cases where i was
- paid a daily rate, found myself working 10hr days for no extra.
- paid an hourly rate via an agency. Asked to work weekends, and i agreed on basis that i would bill for a minimum of 4hrs (in case it turned out there was no work to be done whe i turn up).
- paid a daily rate. Workload increased, eventually asked to work weekends. Agreed to be paid an normal rate, and to be paid for additional hours worked (early mornings/late nights).
(The client/agency/contractor relationship can complicate it a little - the client may have to pay a premium to the agency for hours worked over 40pw. Sometimes a verbal agreement for time in lieu with a few extra hours thrown in can be to everyone's advantage).
I think you just have to take the hit on this one, and make sure you negotiate on the next occasion.