Came across this thread accidentally and although I see it is old I thought it worthy of a reply...
I believe your accounting is correct and the suggestion in your 2nd para is indeed wrong.
I had been filing trust returns for the bank interest exactly as described but at one point had the need to contact HMRC about a late return and I ended up with a letter from them confirming that no further trust returns would be required providing the taxable income stayed under the trusts standard rate (20%) band (£1,000) [and no other changed circumstances]. This was back in 13/14 and with the bank interest already taxed at source at 20% that made massive sense and we stopped returns.
I have recently contacted HMRC Trust & Estates helpline to ask the score now that the bank interest is paid gross and was pointed at their Trust and Estates Newsletter of December 2017 which reaffirmed as ongoing (for now) the fact that no returns are required providing the tax liability would be below £100. So, a deminimis there indeed is. With measly interest rates I guess this is likely to cover many or most service charge situations. The officer pointed out that this applied only to bank interest (fine).
This new relief originated in April 2016.
I hope this may be of some assistance to someone.