Fill the driver's sill with loft insulation. Pipe lagging is way too difficult to get into the cavity and feed it onto the pipe. You'll need access to both ends of the sill cavity to do this, but it's an easy mod.
Last winter I stripped the heater box and sealed it with RTV, then replaced the crappy metal hose (from box to cabin) with a silicone & stainless kit (ebay). No big difference, but I noticed that when tooling around at slower speeds, heat eventually trickled from the vents.....however, when at higher speeds (I get a 20 minute motorway blast to and from work each day, with few other road users around.....) all heat disappears. This suggests that the air passing over the car is cooling the heater circuit to the point that there is no more energy left in it to transfer.
I agree - the heat needs to get to the matrix in the first place! Lagging is a bit of a pain but do-able..
http://www.vx220.org...ng-improvement/