Your only entitlement to a break is to a single unpaid break of 20 minutes if you work for more than 6 hours in a shift. The entitlement is neither 'cumulative' nor 'pro rata'. e.g. if you work a 24 hour shift, you're still only entitled to a single 20 minute unpaid break.
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10029451
To the best of my knowledge, there is no legislation specifying limits on driving hours for those who drive vehicles which don't require a tachograph to be fitted.
You don't have to be provided with a written contract, per se, but your employer is obliged to provide you with a 'written statement of employment particulars' within two months of you starting work:
http://www.direct.gov.uk/en/Employment/Employe es/EmploymentContractsAndConditions/DG_1002790 5
If you work 'overtime' (as you're effectively doing if you lose part of your lunch break for other duties) there's no statutory right to be paid for it. (As long as your average hourly pay, for the whole week, meets the National Minimum Wage, the law is complied with). However you can't be obliged to work overtime (with or without pay) unless your contract specifically requires it:
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10028439
Chris