Baxi duo boiler timer not working since new radiator fitted

Hi,
I'm looking for some advise. I have a Baxi Duo-Tec Combi 28 HE boiler. It was all working fine. Then I bought a replacement radiator for the bathroom and the plumber fitted it, and he said he gave the Boiler a quick service. He is off on a 2 week holiday not so I can't get him for a bit.
Since he left the timer on the boiler no longer works in that it turns the heat on when set to, but the heat doesn't go off when meant to via the timer... it just stays on and on.....
If I turn the timer switch from on to 0 (off) this makes no difference.... the heat stays on.
The only way it turns off is if I turn the boiler off (on the main switch with on, 0 and R on it). So turning it to 0 turns the boiler off. Then when I turn this back on the thermostat timer works again.... But only on timer to turn heat on. It still won't turn off when it's meant too.
Am I not giving to boiler long enough to go off? Even several minutes after the timer is meant to have switched the heat off the boiler is firing away.
There are no obvious faults.
The new radiator was just a towel rail changed to a proper bathroom radiator, and it is great.
Any ideas?
Thanks

How can ANYONE give a condensing boiler a quick service?
Pray tell as I would like to know this magic scheme and increase my turnover

So you buy the radiator, plumber fits it and services the boiler as well. Nothing wrong with that plan

Get the plumber back as he may have done something while he did a quick service. I hope he did not do a quick exit after the quick service

He was an oldish fella. Nice old boy. Charged £75 to take the old one off and put the new one on, and it took him a good hour and a half.... probably due to his age. Seemed a trustworthy type. Think my girlfriend knows him too. He will def come back.
Seems impossible for fitting a radiator to be linked to the problem but never had it before.
I think he just added water to the system or something as the new radiator needs more..... If that is possible.....i know nothing about boilers

Lovely old boy! All that for £75........
Send me his number I could use a guy like him to earn me some dosh.

Did the 1.5 hrs include the “quick service”. ?

Is it a mechanical (as opposed to digital) timer

can you post a pic of the display when the boiler is running when you have the timer off so we can see what is happening