jh5speed.
True all Darts are single spool but I think the ones on the Viscount 810's were -525f's with three turbine stages. I worked on some back in the 70,s. Earlier Viscount 800,s had -515,s with two turbine stages.
When the throttle is advanced the Prop starts to govern at 11000 engine rpm and if the fuel is not being increased at a sufficient rate the speed will decay a bit.
As to Water Methanol, this only comes in at about 14500 rpm if the system is armed and operates at take off rpm (15000) until first power reduction to 14200 rpm. (or the W/M runs out).
Also the -520 series Dart, on Viscounts and F27s, W/M restores power at hight ambient temps so would not be used in the UK very often except on hot summer days, where as the -532 on the Dart Herald boosted power at all temps. so was used more often.
Hope this helps.