If you want to clear £100 per day add on £50 for the taxman then VAT at 20%. (100 + 50)x 1.2 =£180 to start. Van costs and replacement tools need to be covered as well. Work for less and you will have many happy customers, but when the taxman knocks at your door your happy customers won't be paying his bill.
joni os answer is brilliant!
Start with this.
You need to earn what you need at home AND cover your business overheads. Here are some suggested figures for business overheads, real figures could be very different:
NOTE: really rough ANNUAL figures:
Business overheads
£300 accoutant
£400 public liability insurance
£ - ? are you GSR?
£ ? - membership of anything?
£2,000 advertising
£5,000 diesel in the van
£3,000 van maintenance, MOT, TAX, repairs, new tyres and depreciation (saving to buy a replacement - perhaps this figure should be bigger).
£400 tools replace/ repair/ maintain.
£100 stamps & stationary - sending out your quotes/ printed invice books etc...
£100 towards computer/ printer stuff
£500 phone
Rough total business expenses for sole trader: £11,000.
So lets say you need to earn £1,000 a month to cover your business overheads!
My figures might be way out so fill in your own.
So if you marry the reply from joni os and my figures for business overheads :
if you need £100 per day for your self x 5 days a week x 48 weeks in a year
Then you need to charge
£180 + £50 per day (overheads)
We pay various subcontractors (not plumbers though) and we usually pay around £120 to £180 - excluding VAT. I would expect the guys to charge the public more than this.