Switchmaster is a brand owned by ACL Drayton (part of Invensys). Their Drayton Lifestyle range is very good and will do what you want though it looks like the terminal connections are different. See
http://www.draytoncontrols.co.uk/LPProgrammerRange.aspx
(Don't get anything from the LPSi range as it is booby-trapped to stop the boiler working properly at intervals, to make you get a serviceman in. This is not a joke.)
Keep a copy of page 2 of their compatability chart which you can download from
http://www.draytoncontrols.co.uk/uploadedFiles/Drayton/Compatibility Guide_DS_ENG_Aug10.pdf
I find their customer service desk very helpful, and it is a common question for them. Obviously they want to sell a new one from their own range, but it is pretty good so this should not be a problem.
I'm not sure if the SM400 fits onto their standard wallplate (if it does, you will find a small screw in the middle of the underneath. If you turn off the power and undo this screw, the programmer hinges forward and pulls off (pull the bottom out first) and a new one plugs in and hinges back. However this depends on the terminal connections being the same, and it looks like the SM400 is different.
You need to work out if the boiler flow to the cylinder is pumped or gravity. Gravity is common on old systems and is controlled differently (it is slower and less efficient to heat the cylinder). If you post some pics of the pumps and valves around your boiler and cylinder we may be able to tell.
If you decide on another major make such as Honeywell, they will have a similar compatibility chart. Programmers all do about the same thing so you will not usually need much or any change, but the terminals might be laid out differently so you need some ability in understanding and connecting wiring, unless you get a plug-on replacement that fits straight on. If you are not good at this sort of thing any competent heating engineer will do it easily, but make sure you get someone local who has been recommended.
If you doing it yourself be aware that all the wires and connections are at mains voltage until you have cut the power and tested for dead.
Usually you can get a 24-hour, or a 7-day programmer, or a 5+2 if you get up late at weekends, or various other options, such as turning the heating off for 2 weeks when you go on holiday, or keeping it on till 2am when you have a party. I find I need the instruction booklet to hand when setting digital programmers as they can be quite complicated, especially the 7-day one which can have different settings for every day of the week. Heating engineers tend to look down on the simple ones which are cheaper and easier to use.
BTW your old Switchmaster, if in working order, will be treasured by some elderly user, or someone with poor eyesight, as they are very intuitive and easy to set and to read,
