No it's not difficult but no point in starting at 23:30 if the tarriff doesn't change until 00:00.
Sure - but it just needs some thought, and a little (but, I suspect, very little) 'gambling'.
If the immersion is supplied by a 'dedicated' time-switch from a supply which is not switched by a 'tariff-controlled contactor' then, with remotely-controlled tariff changing (e.g. teleswitch) there is always a theoretical risk that the tariff will switch to 'full price' for a period (theoretically anything up to 2 hours, with a 22:00-07:00 'guarantee', as mentioned by Simon) whilst the immersion is active, but I would think that is very unlikely to happen 'in the middle of the night' (since that is likely to be a low demand period).
Hence, in my case (never needing more than 3 hours of primary operation), if (with a "22:00-07:00 guarantee) I set the 'on-time' of the immersion as, say, 02:00, or even 03:00, (with 'off-time' at 07:00), that would almost guarantee that most/all of its usage would be at cheap rate (with a very small risk that some might be charged at 'full rate'), whilst allowing brief top-ups right up to 07:00.
In passing, never having had a remotely-switched tariff, I don't know, but I'm not sure that Simon's mention of "7 hours between 22:00 and 07:00, in no more than two blocks" is necessarily typical. With my locally-switched E7 (pre-programmed non-'smart' meter), the off-peak period is 00:30-07:30 GMT (hence 01:30-08:30 BST in Summer, since the meter doesn't understand the change to BST!)
Kind Regards, John