This had been stored in a shed over winter...
It was 2nd hand when I bought...
There are a couple of members here who know your e-bike quite well (I don't), but from what I've read, troubleshooting guides and wiring diagrams you won't find. It depends which model you have as Halfords changed the Vengeance quite a lot after 2019. The Suntour HESC system is a bit of a dark art (the controller is potted in resin on some models), and some members have resorted to replacing Suntour controllers/displays when problems arose if yours is one of those models. Again from what I've read, it's really only Halfords that will have the majority of original parts for what is a Halfords exclusive model.
If the bike/battery was second hand, and left unused in a damp, sub-zero shed over winter, you may want to first test the battery really is still okay with a multi-meter (should be >41v) - you can't rely on the battery LEDs. It may also be an idea to take the bike indoors to let any residual damp/condensation thoroughly dry out too. Hopefully nothing electrical corroded causing bad connections, but IMHO that may be a real possibility.
Others may be by shortly with pearls of wisdom on where your Vengeance problem(s) may lie, but it'd help to know which version/drive system yours is.