Bit more playing, have wired up the other few displays I currently have and got them working
ended up looking thus
here set up for two tracks (idea is a few more rows of displays for more tracks and a row for the "best lap overall" at the bottom). left to right we have the car number, idea is to be able to show the cars in either lane order or actual race order, and switch between them so whatever is easier to read can be shown. then you have your most recent lap time, followed by your best lap time and finally the number of laps completed - may also swap that to be number of laps remaining and save the count up for endurance racing over a fixed time period.
this is still using a button to count the lap increment points which is not ideal but got it this far
after
much mucking about trying to solder down a surface mount integrated circuit with a truly terrible (but cheap) soldering iron (tip keeps coming adrift slightly so the very tip cools down), I gained the ability to use a Schmitt trigger to allow me to use an infra red LED and phototransistor to create a simple 'beam' sensor which I intend to embed below the surface of the track to count the car guide blades.
currently just a single sensor on wires
it measures when interrupted, and works quite nicely.
next up is to make a few more of the sensors with the aim of cutting the track possibly Saturday morning borrowing Colin's drill and Dremel to try and get a few sensors mounted and test the thing before my Normans are obliterated in the afternoon.
also need to get a few more displays ordered and figure so way to get the displays mounted to something for now