My roof is made from King post trusses and the new floor is going to be
entirely separate, as it I'll be raised an extra 1" off the wallplates.
The
trusses are on a 600mm spacing, and each one has a king post in the
middle.

You can place the new joists adjacent to the existing and bolt them through
so that they no longer need support in the middle. Before removing the king
posts, get a structural engineer to agree that they were only holding up the
old joists, not the roof!

The main disadvantage of this method is that it isn't quite as good on sound
insulation. This is usually only significant in flat construction, though,
rather than houses.

You'll need to examine the roof construction to see if any additional
strengthening is required to the rafters or purlins, especially if you are
intending to add any weight to it, or intend to remove any additional struts
such as any diagonal ones to the purlins.

1) Is it possible to modify trusses? I have searched around t'internet and
some sites say "absolutely not", some say "yes, if a structural engineer
does the calcs for the modifications"

Anything can be modified.

2) Assuming "yes" to the above, is it the sort of work I can do myself (I
am
a pretty competent DIYer)

Only you can know your own competence.

3) Is there generally enough redundancy in the trusses so that one can be
modified at a time While the trusses on either side take the temporary
additional load

Yes. However, if you bolt the new and old joists together, any pure king
post becomes immediately redundant, so this isn't actually relevent.

Christian.