I'd be very much keeping the 0.9% tracker. Rates may go up a bit further, but they will come down too.
Banks tend to average 2%-2.5% over ECB on average over a period. The current anomaly where some fixed rates are attractive won't last and when the fixed rate ends you are at the mercy of their rates then and start are not known for any sort of value and you could be 4% or more above the ECB then.
As for the BTL - the bank manager was advising you based on the situation at that time. He was probably right at that point as no-one foresaw the massive crash and funding issues which led to trackers then being so high compared to the crazy days