s/e, you should invoice him each week. something like :
1.xx hours @ £xx/hour................................ £---
2.allowable expenses (e.g. fuel)................... £xx
3.less 20% deduction on labour.................... £xx
4.amount to be paid ((1+2) - 3 ))...................£---
1.xx hours @ £xx/hour................................ £---
2.allowable expenses (e.g. fuel)................... £xx
3.less 20% deduction on labour.................... £xx
4.amount to be paid ((1+2) - 3 ))...................£---