Thanks in advance
what have i done wrong?
Your wiring looks correct for the last light in a UK radial lighting circuit wired with the connections in the ceiling rose.
- Check that all the screws are fully tightened down.
- The circuit-breaker in the consumer-unit has not tripped out (or fuse blown). This might not always be abvious in daytime.
- There is 240 volts at one of the red wires.
For suitable voltage tester, see When doing electrical work, what do I use to check wires are safe?
Obviously, you need to take extreme care when working in the vicinity of live 240V circuits.
There are three common ways to wire lighting circuits in the UK (in order oldest method to newest)
- wiring is connected in junction boxes under floorboards or hidden in the loft/attic/roofspace.
- wiring is all connected in the ceiling roses (as yours appears to be)
- wiring is all connected in the backbox behind the switch.
Yours might be like the last but with two lights controlled by one switch. In which case the wiring you show would be incorrect. Since you rewired like-for-like this is very unlikely.