I'd replace the video cable first. It's possible it's just a pinched cable that is now damaged. It's also a cheap component to replace, and if it doesn't resolve the issue, then the display panel would be next. So, once you replace the video cable, don't bother reassembling the unit until you've tested it and verified the issue seems to be gone. Otherwise, the display panel is about the only component left, so you can save yourself some effort later just leaving it disassembled.
And it is possible an inverter is DOA, but by and larger inverters just don't go bad all that often. That 14 quid price is a bit steep, so I guess it's up to you if you want to try another one or not.