1. Do I need to change reduce the MCB to a lower ampage. For example,
to 20amp (and how is this done?). If I do change it to 20amp is the
6mm cable (I think) still suitable?

You can keep the 32A MCB. I would, especially for a kitchen, where there are
many power hungry appliances. You can spread the load by taking something
big (i.e. washing machine) off the old ring and sticking it on the new
circuit.

If you do change to 20A MCB, there are no problems with the 6mm cable. You
are allowed to use bigger cable than necessary, if convenient.

2. I'm likely to place a junction box or my first socket at the end of
the circuit. To extend the circuit do I need to use 6mm or is a lower
rated cable OK (for 2.5).

This depends on installation details. 6mm (or even 10mm in some rare cases)
may be required, but calculations will probably allow 4mm, depending on
installation details. (i.e. a short distance clipped direct is allowed, but
buried in insulation or in conduit may not be).

3. Is their a rule on how many sockets I can add and how far apart
they need to be.

There are some limits to do with floor space, but they simply wouldn't apply
for a kitchen. You'd have to be covering the area of a small size house to
run into them, not a single room.

You can have as many sockets as you wish. Indeed, the latest on site guide
gives guidance on how many sockets might be considered reasonable and you
need quite a lot to meet their suggestions. The IEE are on a mission to
reduce the number of multiway extension leads.

Christian.