Always causes a debate.

It does depend on the particular boiler.
Manufacturers maintenance manuals will either say you need to be ‘qualified’ (ie Gas safe) to remove the cover, or they will say that you should be ‘competent’ (ie. know what you are doing) to remove the cover.

The ones that need it to be a GS qualified engineer that removes, or more importanly correctly refits, the cover usually have the timers, electrical wiring, etc. accessible/replaceable without removing it so the question doesn’t arise.

(On my Vailant combi boiler the timer unit just pulls off the front, with no need to remove the cover or even unscrew anything. It takes literally seconds to replace one).

If in doubt then get (pay for) a GS qualified boiler engineer to do the job.

If you have a few hours to spare have a play with heatpunk.co.uk . It is free to register and you can “design” your house in their online software. You will need details of the fabric of the dwelling such as the type of walls and the amount of insulation. When you add details of the radiators it can calculate expected losses from each room and the expected output from each radiator. The software was designed to aid the transition to heat pumps, identifying potential weak spots in the heating system. But the basic design calculations are good for any heating system. You can dial in the “heat pump” operating temperature (the water flow temperature) and it will show if the radiators are adequately sized or too small for each room. So you can work back, try different flow temperatures until all rooms are adequately heated. That will give a starting point for the optimum boiler temperature.

If the dwelling feels OK fine, otherwise you can tweak the temperatures.

If the software shows a flow temperature of 55°C is OK then that is good news if you are thinking of moving to a heat pump in the future.

Ah ha.. Tomas (YouTube link above) was using HeatPunk in one of his videos, it did seem quite laborious. Have you used it ? I might give it a go just for fun - assuming it is

Meantime my friend is trialing a low flow temp (1.5 on the dial) and a lower hot water temp, no visible water vapour at the flue today, prob about 2 deg outside. House comfortable so far. He’s on Eon and in spite of having half hour data on his smart meter their online info doesn’t give more detail than daily usage data ‍♀️