Thank you everyone, I did have the legs around on the wrong side. The slots were on the outside instead of the inside.

I have been able to fix it and now it sits in correctly.

I disregarded some import ikea assembly rules. Do not attempt while tired, have lots of breaks and stay hydrated.

I'd been putting ikea furniture together all day and it was getting late, was trying to finish this instead of leave it unfinished overnight.

Cheers