logics are a very good boiler, only issues i found is following so called engineers who service them without even opening the case, this is the same with all other boilers aswell.
is it a combi for combi swap?
to many things influence the cost apart from the boiler itself even if you had a not very good boiler like a vokera....... gas upgrade, powerflush, controls, filter, flue system, condense route etc......
Did the powerflush take all day? What chemicals were used? What pre and post survey paperwork was left? British gas won't guarantee much after powerflushing but they should do a proper pre and post survey at least.
Sometimes the only solution is to renmove the badly blocked paperwork.
no idea of what hourly rate you would pay, remember an hourly rate is normally based on a full days work.
you will need the gas safe engineer to disconnect the hob if your changing the worktops/ kitchen which will involve another visit.
if the vessel just needs recharging then the cost should be 1 hours labour. this can be done on an annual service so maybe book a service if it has not been done in last 12 months but tell the engineer theres a pressure loss issue.
tell her to ask the engineer who does the annual service to check it out. as above its possible an expansion vessel, prv issue. if she has not had it serviced for a few years it probably needs an important seal changing.
just measure the distance of the current tail on the right, divide it by 2 and get two tail extentions that size and refit. may have to alter the brackets. speak to council sparks about the bonding being reconnected.