So. I've spent an hour or so on it and discovered that the only spare forks I own have a 1" steerer. Somehow I've bodged them securely into a 1 1/8 Hope headset - which has a huge stack height - therefore I have no spacers and need a riser stem.
The chainset and BB have been stripped from a old hybrid frame that will be going to the tip. I was tempted by a shiny one at £45 from SJS but have resisted the urge to "click here!"
Both of my spare seatpost clamps are too big, drat. I don't see any way of making them fit. That brake is a Magura hydraulic. I'll be using a pair of old ceramic rims which scared me to death with normal brakes.
I only have 19 and 18 toothed fixed sprockets in the pile... that is going to be hopeless with a 42 tooth front.
I really want to build this without spending a bean! Anyone want to swap cogs? I need a 16.