Thanks for all the suggestions.

I was considering the circular saw method but my blade produces a 4mm slot
whichmight be too wide.

I hadn`t realised that you could use "straight" router cutters to produce slots
as I thought that they would be too flimsy for such a tough job.

Biscuit cutters seem to be one solution but rather expensive and possibly too
thick.

Someone suggested Woodfit to buy the "magic wires" from. I thought they had
gone out of business a couple of years ago so I will have to investigate
further.

I realise that there is a danger in splitting the wood using these
"magic-wires" but I intended to insert vertical supports in mid-shelf to
prevent any excessive strain.

Thanks