Its not in writing. It was some bod from the CAA that discussed it.
The only thing you have to have to be able to use the twilight time is the night instructing restricting removed.
If you have that removed you haven't any worries.
If you start back after night time starts and you don't have this removed the student all of a sudden turns into a pax when the bell tolls. Then you would need to comply with the 90 day rules with one night landing if you don't have an IR.
You can actually do the night qualification as part of the PPL course. Most folk who go to the US do it as part of the 45 hours. One lad even did his XCQ at night but he was on a mission to get his 100hours night solo done before starting his cpl. I might add that now I think this was dubious from every point of view.
It is frowned apon extremely by most of the old and bold in the UK. They say that the 45 hours isn't really enough to cover the whole of the ppl course anyway which is true with most pilots who fly once a week with stoppages.
The main reason is that you have just removed 5 hours for the NRQ course which could have been an extra 6-700 quid in the bank. Most PPL's stay interested enough after the PPL to get a NRQ. Its not until afterwards they realise that it is next to useless in the UK unless they are willing to pay heaps of money for large airport landing fee's.
I wouldn't recomend getting the restriction removal though.
Night training risk factor is way high compared to day training and its 4-5 more stressful and tiring than day instructing .
You have just set yourself up for 12 hour + days all year round.
It's a ball ache sitting on the ground supervising the 2 hours solo.
You won't get much of a return if any on your 250 quid to get it removed.
The amount of night hours towards the magic 100 won't really be much over 2 years you will be lucky to get 40.