The hot water was timed to run at certain times of the day, everyday
and circulated the radiators, keeping them lukewarm (I never needed to
turn on the CH) and kept the small flat warm enough. It is wasteful to
heat up a whole cylinder of water just for a shower and washing-up for
1 person. I'm thinking a combi is more suitable in my case.

Absolutely, if there was any installation that is crying out for a combi it
is a single person studio flat with a shower.

Would replacing with another conventional boiler be cheaper than
installing a condensing combi system?

Possibly not. Your existing system appears not to have modern controls (i.e.
rads come on when hot water is required etc) and would need to be brought up
to date. It is probably cheaper to install a combi and remove the old
cylinder than to bring the old system up to scratch.

I am worried about the condensing plume of steam affecting neighbours
in my block of flats.

That would depend on the flue position etc. We couldn't possibly comment
without a more detailed knowledge of the layout.

Please advise me as to a suitable make and size of boiler. I have
looked at Alpha and Vaillant but seems the heat output is way too much
for a 1-person studio flat but what do I know. How do I calculate the
size of boiler I need?

A combi is sized by the hot water requirement, not the space heating
requirement. A studio flat probably only needs a few kW to keep warm.
However, you should be looking at 28kW for the hot water to get a lovely
powerful shower that doesn't go tepid in winter.

I've got a Worcester-Bosch Greenstar. Not a combi, but an excellent boiler
and available in combi form.

Some possibilities ideal for studio flats:

http://www.discountedheating.co.uk/s...unior_28i.html
http://www.discountedheating.co.uk/s...bi_Boiler.html
http://www.discountedheating.co.uk/s...bi_Boiler.html

If you need to route the flue through a long and circuitous path, then the
Keston allows you to do this with standard high quality drainpipes, rather
than expensive flue extension kits. Costs a little more to start with but it
includes the flue terminals (which are extra with most boilers). With any
flue length or with a few bends, it starts looking much cheaper.

http://www.discountedheating.co.uk/s...mbi_Range.html

Christian.