Well £70 per day x 20 days is £1400. Then you need to add their NI which you have to contribute to as an employer, roughly £120 per month. That's now £1520. Then there's employers liability. £16 per month ( £200 a year). That's up to £1536 now. A van would cost roughly £150 diesel per month, £30 insurance, £18 tax and £20 running costs. That's £218 ( tax deductible ) but that now makes £1755.
There are always other things to consider like paying his holiday pay 4 weeks per year whilst he's not earning you any money.
So, as an average to employ someone working on their own theyre gonna cost you around £1900 per month.
£1900 / 20 days is £100 a day give or take a few quid to cover all your costs, before you start making anything from them.