Me and my dad did mine (and he knows whats he's doing), a very long DIY task esp putting a velux in at around £1000-£2000.... is the boiler tank up there? because that would need moving also! there's alot you dont even think of that crops up and starts adding up! you'd be looking at around £5000 for a bog standard refit and I mine bog standard, 7000-10,000 on something good (if you know people in the trade i.e sparky, joiners)