I need to put some insulation up there - but don't want to waste money
by putting something down only to find it unsuitable when we come to
convert.

If you want to start on the conversion, as it were, you should insulate the
roof with Kingspan.

You will need to decide whether you are going for a ventilated area behind
the insulation, or a full fill.

With a ventilation fill, you leave 50mm between the insulation and the felt.
This area is then ventilated using soffit and ridge vents. You then apply as
much insulation as you can (25mm in your case) and then more underneath up
to 100mm total thickness (75mm). Then foil backed plasterboard over that.
(You can order the insulation ready attached). This is a simple technique,
but can make the roof thicker, which may be a problem for headroom,
especially when you consider that the floor may need to be strengthened
(i.e. made thicker).

The alternative, with no ventilation space required is just to fill the
entire space within the rafters (75mm in your case) and the rest up to 100mm
below (25mm in your case). This gives much more headroom. The problem is
that you loss the ventilation and another method must be used to ensure that
the roof timbers don't rot.

Your two alternatives for this solution:

1. Have no felt (old houses may have no sarking at all. The gaps between the
tiles/slates provide adequete ventilation for the roof timbers). I'm not
really suggesting ripping the old felt out, as it provides a useful backup
when the roof covering fails and is required by building regulations.

2. Reroof, using breathable membrane instead of felt. This allows moisture
to pass through, but still provides weather protection. Unfortunately, it
needs to have all the slates/tiles removed and replaced after installation.
This may be an option if you are intending the reroof anyway.

There is one other solution if reroofing anyway and you have a detached
house and no problems getting planning permission. This is to install a
genuine warm roof. You'd put insulation between and above the rafters. The
rafters than become part of the warm climate below, protected from the cold
and wet outside air. It raises the roof, which might not be aesthetically
pleasing and would create an ugly mismatch on a row of terraced houses.

Christian.