After seeing a local racer using one of these I decided to pick one up.
https://lapmonitor.com
After trying it at Bury I was really impressed with how well it works. The transponder is pretty easy to install and the sensor / app just worked, didn't miss a lap.
It can be difficult to feel when your quicker/slower when practising without timing gear.
Even with timing you don't normally get instant feedback, having your lap time spoken to you as you pass the sensor was brilliant.