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Thanks for those comments, all very helpful. Just one point that
perhaps can be answered, how do you come to a figure that indicates the
efficiency of a boiler ?

See http://www.sedbuk.com/

The table on the first page gives a general indication of the savings to
be had and to my mind suggests that on cost grounds it is probably never
financially advantageous for most people to replace a working gas fired
boiler as the undoubted savings in gas costs will not exceed the capital
cost of the replacement (and its probably higher maintenance costs)
before the new boiler with its built in obsolescence reaches the end of
its economic life.

The best condensing boilers (as can be seen on the next sedbuk page) are
marginally better than the table suggests and as your boiler is so old
it could well be 55% efficient (or even less) so yours could be one of
the few cases where new will produce a saving. However the chances are
that as condensing boilers need larger radiators to allow them to
operate at their most efficient you would need to factor in the cost of
the whole of a new heating system or accept that your savings will not
be as great as sedbuk predicts.

No doubt if Dribble is around (I have him killfiled) he will have
claimed you are financially inept (or worse) for not having spent
£2-3000 on a newer boiler long ago, but economics is one of the many
subjects that Dribble just can't understand.

--
Roger Chapman