I was having another look at the lighting last night, to work out if
current circuit would take our intended lighting. Our lighting circuit
is running on a 16A MCB, I expected it to be 5/6 maybe a 10A. Is this
16A MCB ok to leave for the lighting circuit?

No, reduce it to 10A. Most light switches and fittings are rated for that
value, except SBC and SES fittings, which are 6A.

A few people have suggested it may be prefferable to put the kitchen
on two circuits. I was wondering if the boiler circuit which runs
through the kitchen could have some other appliances added to it.

There isn't much diversity left on a 16A radial. I might consider reuse for
the fridge/freezer, but not for major heating appliances like washing
machines, dishwashers or ovens, unless you can fit with 100% allowance.
(i.e. 3A for CH and 10A for a 2kW oven would be OK).

One alternative if it is run in 2.5mm cable (which it might not be, so
check), is to complete a ring with it back to the CU and upgrade to a 32A
MCB. This could then be used as a non-RCD circuit for fixed appliances,
leaving the other kitchen ring as an RCD protected portable equipment socket
circuit. Ensure the CH runs off a 3A FCU, though.

Christian.