Ok well I would ask the following;
Where else have you done work, can you see the installs so you are happy.
How many tubes, and why that number, on what basis. I would say 30 tubes for your area as a rough guess on 250 litre tank.
Will he tilt, to what angle and if not why not. They should tilt them to say 45 degrees or so Facing true north ?? If not north then what orientation and why that orientation.
Do you fully insulate all fittings, at the tank to the tank wall, at the panel so ALL brass and copper is covered (not just the copper pipe), so no loss of heat.
Will you supply a manual on/off switch for electrical boost (if not gas boost) so I can turn the power off for no boost if I require.
Does the price cover all installation, removal and all work so there is NO additional cost, and will you come to site and confirm all costs prior to starting work?
What tubes are you supplying and using (brand and model/type, not just evacuated tubes)
Will you ensure that fitted are, steam vent, cold water supply pressure control valve, cold water supply expansion valve (you may have these fitted already) and solar tempering valve.
What circulation flow will you ensure will be provided on my installation given the length of pipe to and from the panel.
What type/brand of circulation pump and will you test the flow on completion so I have the "correct flow", rather than just making assumptions.
What controller will you supply and is it adjustable or fixed black box.
How will you mount the tilt frame to the roof, what mounting points and how are penetrations through the roof covering done? (Should be mounted to the rafters and NOT the battens as they can pull off).
Will you be using solar insulation for the exposed pipework outside rated for exposure to the sun (armorflex solar).
