Hello cricket, I think there are a lot of variables you are not factoring into your proposed install? Not your fault, I admit.
The chaps on ebay will quote you £900 to supply and fit. But I guarantee you there is a SMALL PRINT somewhere. You will not know about this till you are submitted with the final bill? Yep, it may be as much as £1,900. At which point, you would wish you had used a local GSI to supply & fit the boiler of their choice.
I know of engineers who replace combi boilers for £300. Not me, not a chance in hell.
Peeps ring you and say: ''c'mon, how difficult can it be? You remove the old Combi and replace with new Combi. Easy as chips. Should take you a maximum of THREE hours.''
Really???
Old boilers had no condesate to route to a satisfactory drain off point. That has to be done. When your system was installed, there was no need for every bedroom to be kitted with a TRV. All bedrooms require one now. I could go urn but dont want to sound like a broken record.
For me to replace a Combi for another Combi, you are looking at a minimum of £1,500. That includes boiler, TRV's, flushing system, dosing with Inhibitant, fitting a TF1 Filter, RF room stat etc.
Good luck with your £900n budget. Will you find someone to do it? Yes, plenty will do it. NOT ME