Where can I collect a L Corner 900x900 unit fast !

Have you tried the sheds ? B&Q usually keep the carcasses in stock.

Try Ikea, Brent Park, North Circualr Rd. Have 900x900 corner unit but do check height as could be diferent from some British makes.

I thought of ikea, but remembered that their sizes are unique.. high wise and also how much room is left at the back

Your two choices would probably be

B&Q - which is 925x925 including a 83mm? void behind both side. You cannot use a carousel with this are they have a post on the internal front corner (makes them fairly crap IMHO). Big one in Enfield (A10) , smaller one in Whetstone (A1000).

IKEA - Brent Part NW. Or new one in NE at Picketts Lock. Both of N. Circular A406. Truly 900x900 - doors are 2-off 700x300 and mounted so that one of the doors is hung off the other one. No voids behind.

Dont forget legs? shelves (included on B&Q)? hinges (included on B&Q) & handles.

I doubt if there is more than about 5mm in it for the base units. Which can be easily be hidden.

The wall cupboards are the ones which are different. IKEA do 600 and 900 version whereas others do ones around the 700 mark.