Problem with new boiler install

I have had a new boiler installed (combi Valliant eco) at my mothers house. The plumber tried to fire it up on Friday.....everything was fine, but he couldn't get heat to the downstairs radiators. (Hot water = fine).

The house has two rads upstairs and three rads downstairs. Boiler is now located upstairs. (previously downstairs, but moved due to flu issues)

Plumber is a really nice independent gassafe chap whom I have known for years and totally trust him.

He stayed to 8pm Friday until I sent him home to sleep on the problem. He arrived at 8am today and stayed until 5pm when I sent him home. He has cut through all the new joins he had made to check for blockages. He had already powerflushed the old system, and fitted a magnaclean thingy.

We/he just cannot get heat to the downstairs rads.

Any one got any suggestions?

what system did you have before-was it a combi boiler

Not sure of the name of the system - but I can explain it.

Tank in loft
Immersion tank in bedroom/airing cupboard
Ideal wall mounted boiler in dining room (fitted by British Gas), on the floor 24 yrs ago - so can't complain about getting our moneys worth out of her.
2 rads up stairs
3 rads downstairs

Plumber kept old rads as thought they were OK, but replace old TRV with new ones.

just a couple of things while i put my thinking cap on
.
you said the rads are getting hot up stairs ok
have you turned these two off to see if heat goes d-stairs
when ripping old cylinder out and pipework you have removed old motorised valve and awkward pipe work(only because someone did not this week)
some where you have a pipe drop (2 x pipes flow and return going to your downstairs rads-r these visable - have you felt them
is their a brass non return valve on one of them(looks like a fat ballofix isolation valve)
did your heating drain down and refill with ease

has the system been flushed

Thanks for reply.

a) Yes tried turning off both hot rads upstairs to try and force heat downstais = no joy
b) Yes all motorised valve stuff gone
c) I can see the two drop down pipes (both now lagged and chased into brickwork) - can't feel any heat in either pipe
d) Can't see a brass non return valve
e) yes system drained down fine and was power flushed proir to refill.

System refilled OK with pressure reading 1.2bar

Temp on hot water setting = 60 degress
Temp on rad setting = 60 degress

when top floor rads are getting hot does it happen quickly or slowly
and do they get to desired temperature
have you tried opening d-stairs drain-offs-(with hose of course to garden)open fill loop to boiler-to check flow rate of pipe work on both flow & return to out side
magnaclean flowing through ok
boiler on & off valves (flowing through ok)working ok-sometimes have filters in
downstairs rad valves all open(stupid question)
try upping pressure on system a bit in case of air lock in any high points