I hope you are ready for the £100+ bill when no fault is found.
The trick is to get your ISP to flag / report it as an intermittent fault, that is what I used to do back when we were on ADSL2+.
I used to do that due to our line had got to a point where it would get worse and worse to a point it was dead when it rained or become very windy and when it stopped raining or wasn't windy outside it started to work again.
I know BT was fine flagging it as an intermittent fault and the several times an engineer was sent out (including a boost engineer) we were not charged when they couldn't find an issue.
I know one of the first engineer visits they found a fault in our pair, sadly there was no available working pairs left, so maybe that's why we have never been charged for an engineer callout.