I notice in this weather that the heater performs very badly, despite a good 90 degree engine temperature on a 15 mile run.

How much of this is normal:

1) Regardless of the position of levers, the forward-facing bullseye vents always blow unheated air?
2) The dash vents can be made to blow warm/tepid air, but very very gently even when on fan speed 3?

My car has a switch controlled solenoid valve that controls whether the coolant can flow through the heater matrix. This appears to work correctly but it's hard to know if that's causing a choke point that's causing low flow through the matrix.

I'm guessing that I probably need to take the dash top off and see if I'm hemorrhaging blower air somewhere... 20 year old gaffer tape gone crusty etc.

Thanks

Gary

First thought is an airlock in the heater matrix. Its one of the high points in the system and air tends to collect there and get stuck. Many builders put an inline bleed screw near the bulkhead to help the air out. Mine doesn't have that bleed screw but I've found if I refill the coolant with the back of the car jacked up as high as I can and fill slowly the heater matrix fills up but otherwise it airlocks. It was designed to be flowing the whole time as a bypass so the valve you have, meaning its offline most of the time, may exacerbate the air lock on filling as if you filled the system with it closed it may never fill up.
The other thing to check is how its been plumbed. When I installed a 160 engine from a TF I kept the plumbing as per the TF but soon found the heater loop didn't flow at all. The TF coolant hoses have a separate bypass loop as the TF has a valve in the heater loop so the bypass loop takes flow before the stat opens but I found that means the heater loops becomes the path of most resistance so never flows. I cured that by taking out the separate bypass loop and having the heater loop as the only bypass but if you do that you can't have the valve in the heater loop or coolant can't flow anywhere with the stat closed. If the air flap is shut no air comes into the cabin through the heater matrix so it doesn't get too hot in summer.
I think the eyeball vents are supposed to always flow cold, maybe slightly tempered. The hoses that feed the demister vents do come off the heater box but these are accessed from below in the footwell.
Good luck!

To answer you first question - mine blows proper toasty once the engine is up to temp

Thanks guys. You've got me thinking re: the bypass. This is actually a KV6, but I think I'll start by temporarily removing the solenoid valve, and then also have a feel of the rad hoses after a run, because 'toasty was not a word in my vocabulary at the end of the trip to work at 8am this morning.

Presuming you have a metro heater fitted, the 3 screen mounted should be hot, and two facing the cabin are cold fresh air only.
My V6 is very warm and cosy in the cabin - always have to drive with the window ajar to allow some air movement.
Tbh I do have an after market heater matrix fitted which causes the issue of all vents blowing warm air so the cabin gets too hot - I'll swop some heat for your cold if that helps as I need to find the opposite and how to make the cabin cooler on long distances
It also warms up very quickly so maybe not enough flow???

Yes just the standard metro heater and blower. I think you might me on to something with the flow.

If the airflow into the cabin is slow, even on fan speed 3, it is probably due to losing air from the blower. Mine used to be bad around the junctionfrom the blower unit into the cabin. Closing up any gaps around there and also to the screen vents should help to maximise the airflow. If both the heater pipes are hot so you cannot hold them then it is also worth making sure that the temperature lever is moving the distribution flap fully - mine only used to get warm rather than 'toasty' and it was partly caused by the teparature flap in the heater not opening and closing fully. Hope that helps