Well done Dizzy - great time and a finishing sprint!! 
caper - I've never even tried butterfly - always been too embarrassed but a bloke recently swam the channel doing it so he might have some tips 
This is a geek alert - I have geek-weeks where i spend a lot of time reading swim forums, watching videos. This week I've been using my tempo trainer. It's a bleeper that you put under your hat and it bleeps at set intervals so you can take a stroke
www.swimswam.com/the-nitty-gritty-of-stroke-rate-tempo-trainer-pro
So I was trying to find my 'cruising speed', a speed that i could keep up for a long time at 19 strokes per length with little effort. So i swam 100 relaxed lengths and then matched it to the tempo - I think it was a stroke every 1.8 seconds. Now I might use the TT under my hat when I do bigger swims so I don't panic and start windmilling and get knackered.
They are good for increasing your stroke rate (and speed) but you are supposed to slow the tempo if your stroke per length increases massively as this indicates your stroke has broken down. You can also check your balance in your stroke by slowing the tempo to 2.00 seconds per stroke or something (mine is a bit crap). This also looks a bit odd if you are swimming in a lane as your arms tends to hang in the air while you wobble. It's quite relaxing though.
Finally, you can do pyramid training, where you set it at a reasonably fast tempo and go slower then fast again over either 25, 50 or 100 metres. So set the tempo at something like 1.3, 1.4, 1.5, 1.6 then 1.55, 1.5, 1.45, 1.4, 1.35, 1.3 - you might find you take fewer strokes on the last 1.3 than the first as the slower ones you have done will have reset your brain to take longer strokes.
THe idea eventually is to have various 'gears' you can use when swimming different distances but I'm a long way off that yet.
Anyway, just thought I'd put it out there as they can be 'fun' to play around with and make the lengths go by quicker. I've just learned to change the digits on mine through my hat instead of taking it out...