Also find out if you're being classed as a subcontractor under the CIS system.
If you are, they will deduct 20% or 30% tax when they pay you which you then enter on your SA to the taxman.
If you are being treated as a subcontractor, what ever price you thought of, add 50% and quote that.
For some odd reason, companies, when they subcontract expect to pay more than if they simply pay for the job done.