Thanks all. Few comments on your thoughts
1. Thanks alan_m for your tips. I forgot to mention there will also be an 18mm "worktop" on top of the cupboards which I could screw down into the cabinets is needed. The only real weight is a 450mm cupboard which will sit on top of the left half of the 900mm cupboard but on top of the worktop. The left edge of the upper cupboard is on top of the side of the lower cabinet but the other side is on top of the middle of the lower cupboard.

2. Thanks John for your tips.
- Yes have a circular saw so could do a groove as you suggest. Might even be able to do it using my track saw
- Good idea the clamp wonder if making a right angle out of 2 no 4x2 to butt against the sides would work better or maybe it doesn't matter? Was thinking, would I need to clamp top and bottom to ensure both are square? Not sure how I would clamp both at once though as somehow the cabinet would have to sit on the bottom one?
- the hinges, I was hoping to use flush hinges to show it was hand made and then have a clip/ catch pinned to either the top or the base to clip the door closed.
- gluing the 1" to the front of the 12mm shelf, do you mean cutting a 1" strip out of the 12mm MDF and glue it with the top of the 1" edge glued to the front edge of the shelf (tops flush)?
- very kind of you to offer to draw it thanks very much but I think I have it - famous last words

Thanks again

Lee.