This is a message I posted in November 2013 :

quote Just had a new hot water tank fitted (not brave enough to DIY)
and it is fine. Hot water is hot etc., but hot water now takes much
longer to reach the taps than it did with the old tank. The pipe work
all looks the same, so is there any logical reason? Tank and boiler
stats have not been moved. Not a problem as such - there is plenty of
hot water, and at the right temperature, it just takes a long time to
arrive. The old tank would have been at least thirty years old. Have
tank designs changed? System is entirely conventional. Oil fired boiler
feeding hot water and central heating. Not a combi boiler. unquote

Since then, nothing has changed in spite of plumber coming back saying
nothing wrong, me forcing cold water back through the hot water etc.

Yesterday, a couple of guys came and changed four radiators, fitted TRVs
and removed an old bath, basin and toilet, capping the pipes. Much
turning off of hot and cold water, partial draining of heating etc.
Today, the hot water is back to how it was before the new tank was
fitted in 2013. What have they done? Whatever the problem, they have
fixed it. All I can think is a valve somewhere was only partially open
(although flow was fine), and yesterday it was fully opened.

Delighted, but flummoxed.
--
Graeme