My astra j 1. 6 2010 starts okay but there is no power at all it won't rev and is breathing threw filler cap

Sounds like back pressure in engine , check engine breathing . May also be a partially blocked cat / exhaust

I've unbolted the cat and tried that it still seem to still be breathing threw the engine I took the filler cap of and seemed to change the pitch still wouldn't a code come up if the cats blocked thanks in advance

On most engine systems if you remove the oil cap the engine does change due to the set up on todays cars . Would have thought you would get fault codes if scanned with a decent scanner with TECH 2/3 capabilities or similar . If that year / model has a timing belt as opposed to a chain , could be worn , stretched , jumped teeth and be miles out throwing timing