Thanks for the responses so far.
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For the Vaillant:
Drain down, strip out and dispose of existing conventional boiler and flue. Installation in existing location of new top of the range, market leading Vaillant 415 ErP Ecotec Plus conventional boiler and new Vaillant horizontal flue. Link boiler upto existing services i.e. flow and returns, gas feed and condensation pipe. Boiler to be wired into existing system controls. System chemically cleansed and inhibited on completion. Boiler and system fully commissioned on completion.
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For the Baxi:
Drain down heating system,take out old boiler and flue.Supply and fit new Baxi Ecoblue advance 25 and flue in cupboard.Take flue to outside and make good.Connect up condense,connect gas and test,pipe up boiler flow and return.Supply and fit in airing cupboard new magnaclean filter.Wire up boiler and new programmer.Fill and clean system,flush out and add inhibitor.Refill and test out fully.Make good affected areas,clear site and register boiler with Gas safe.
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For both of the quotes I've also asked for consideration re: getting a new controller as well as my current one doesn't have different day settings for weekends.