I got hold of a Punkt cutter for £10 on eBay, started to wear after 30 or so cuts. You'll know when it starts snagging and your wrists hurt. Not sure if wet cutting helps, but the blocks are like sponges, when they get wet they stay wet.
Ended up with 50 arches, making them rise at a 4/100 gradient while keeping the arches lined up is best avoided, my brain hurts. The left hand side of the viaduct I gave up through boredom and layed a concrete base on a 4/100 slope into the gazebo instead, then mastic'd them into place on top. Seem solid enough.
I got sheets of OSB, cut loads of 8ft long 4" wide straights. Got the tape and string out to measure 60" radius curves for my tracksetta gauge. It was then simply a case of felting them and screwing them all down. Tracks going down over the weekend, then need to work out how to waterproof the point motors on a passing loop.
Eventually I'm going to put a Wye junction in to get into the back of the garage, probably next year now.