Thanks for our advice - a few Q and A's on what you said..

Do you mean there is no wall in the loft between each terraced house?

Yes

First fill the gaps between the joists with insulation. On top of the joist
install slabs of thick ridgid foam.

I like this idea, but...bearing in mind that I will be walking on this
to a computer that I intend to permanently site up there, will the
foam compress under the boards or spread the load effectively enough?
I imagine the foam would take up some uneveness in the joists?? - this
would be good, as being an 1863 house, the joists are a bit up and
down. I guess the noise will be reducred too? And the foam weight is
negligible as I want to minimise the extra weight involved when
boarding out to maximise what I can safely put up there?? What foam do
you recommend and what thickness - is there a good value brand
available (do you mean Kingspan type stuff?) as I'm on a bit of a
budget for this?

I intend to remove the filthy old existing insulation and replace with
new. This process will involve vacuuming out the years (inches!) of
accumalated dirt before I start. Does anyone see and problem with this
approach?

Also how do you approach insulating deep into the eaves - I thought
there was plastic moulded formers available to finish the insulation
into the eaves (triangle-shaped which the ends of the insulation run
into), but cant find them anywhere on the web.

On top of that the boards. *The secews
go through both and into the joists. This elimintes cold bridging through
the joists and increases the insulation thickness without using extra
joists.

Any ideas for board material, stronger and lighter than chipboard (to
reduce the stress on the joists/roof?). FYI the ceiling joists are not
resting on the wall plates, but are constructed so that they are fixed
to the (substantial) rafters which sit on wall plates. This means
there are sloping edges to the ceiling that I havent yet worked out
how to insulate behind the lathe and plaster, due to lack of access -
any ideas how to finish these?

It is best to lay a vapour barrier on the loft floor above the plasterboard.
This is poly sheet. This will prevent water vapour entering and condensing
in the loft. *Also seal up any pipe and wire holes in the loft.

Do you mean so it goes up and down over the joists, or just strips
between the joists? (presume the former)

1. *Extending the wall up into the roof space is best using lightweight
blocks. On the top of the existing walls use a course of highly insulated
blocks to reduce heat loss through the wall into the cold roofspace.

Any brand recommendations?

2. *Use wooden or metal studding and Fermacell boards. These boards only
need one layer and will conform to fire regulations.
http://www.fermacell.co.uk/

You may want to insulate the sloping roof having gone this far. Make sure
any air gaps are clear.

Any thoughts for v. lightweight boards to go over the rafters
insulation - hardboard possibly??

You mention soundproofing. *Why? *These walls are in the loft.
Soundproofing will be on the loft floor via the insulation and boarding.

Totally agree!

Many thanks for your ueful post!