To the original poster,

Have you thought about replacing the timeswitch AND the wall thermostat
with a Programmable thermostat?

In your existing system, you have heating off vs Heating on at XX°C.

You have no way of having different temperatures during the course of 24
hours or over a week to suit the lifestyle of the family.

I have 2 programmable thermostats, the SALUS ST620, one for upstairs
zone on 7 rads, and one for downstairs on 7 rads

At the moment i have it set to:

heat house to 18.5°C from 6.00am to 7.00am We get up at 6.30am and
depart for work at 7.30am.

Heat house to 5°C from 6.30am till 5.00pm. No point heating house when
we are all at work, but we want frost protection at 5° to avoid burst pipes.

Then we have heat to 18.5°C from 5.00pm till 10.00pm. I leave work at
5.00pm, arriving home at 5.30pm.

Then heat to 14°C from 10.00pm to 6.00am Upstairs only and 5.0° frost
protection downstairs only as then we are all upstairs in bed, covered
by duvets and cuddling wifey and there is no one downstairs.

The Salus ST620 are only 60 quid and are very flexible.

It has a daily mode, a weekdays (5) and Weekends (2) mode and a 7 day
week mode. You have can up to 4 set points per day.

It also has a holiday mode so it then turns into a frost protection
thermostat when activated before you go on Holiday.

My last house was a 2 bedder (Which I still own and now rent out), and
my new house is a 5 bedder. I pay less in gas each year for my 5 bedder
than my tenant does for his 2 bedder.

The Programmable thermostats allow you to use gas in a much smarter way
more closely matching ones family's lifestyle.