you could probably fit it yourself from the instructions, if you are reasonably handy. You can probably use the old screw holes. Avoid drilling new holes above, below or beside electrical fittings as there may be cables in the wall. It is essential to turn off the power to the boiler before you start, preferably at the main switch in the consumer unit, because mains voltage goes to the thermostat. It is best to test for dead with a multimeter. A neon screwdriver is not a safe tool and belongs in the bin.
Unpack it and identify the terminals first. You will need a medium and a small screwdriver.
Photograph the connections on the old one as you work, in case for any reason you have to put it back.
The first time you set the timings and temperatures will take you a while. Decide on them and write them down before you start.
Have it turn up the heat half an hour* before you normally get out of bed or come home; and go down half an hour before you go out or go to bed. Unlike an old timer, it does not have "on" and "off" periods, just "warm" and "cool" ones. During the cool periods it will not come on unless the temperature drops below your setting. You can turn it to12C if you like a cold house, that will be enough to prevent pipes freezing and keep it dry, for example if you are away for a winter holiday. Look at Holiday setting and you will be surprised at what it can do. I usually put mine on holiday during the summer months.
*if you have a badly insulated old house and small radiators, it may take longer. With experience you can modify the timings. A modern well insulated house is unlikely to get as low as 15C overnight if it was warm in the evening, so the heating will act as if it was off..
For example, you might like:
Monday - Friday First: 07:30, 19C, second: 09:00, 15C (if you are usually out during the day), fourth: 16:00, 20C, fifth: 22:00 21C, sixth: 23:00, 15C.
Saturday and Sunday can be different. You can program any day of the week with its own times and temperatures.
IIRC you can program it before you fit it to the wall, running on its batteries. Lithium batteries will last longer than alkaline.