I thought I should give an update, so anyone with similar issues knows what to do next.
I got some new shanks from Craftbrewer. They are different to the originals in two ways:
1) The inside surface is meant to be better quality and is currently nice and silver and smooth.
2) The chamber behind the faucet is smaller, so less beer sitting idle, and perhaps a bit more consistency with liquid flow.
Changing the shanks on one of those 3-tap fonts is, well, lots of four letter words. The spanner provided with it, for undoing the nuts inside the tower for the old shanks, is a RING SPANNER. Think about that for a second.
I actually bought a new faucet/shank spanner with the shanks, which is a tad better, but the old nuts were too big for this.
I eventually got the new ones screwed in tight using a combination of the new spanner and a spanner from my toolbox, a fraction of a turn at a time with each one, alternating between fractions...
Anyone who has done this before would know my pain, anyone who doesn't have one of these fonts is probably wondering what the hell I'm on about.
ANYHOO with that done, I put everything back together, and I now have a tube going up to the top of the font to deliver cool air from a fan in the fridge. The top of the font has some nice condensation on it and the font feels pretty cool. Unfortunately by doing this I sacrificed the fan's previous role of circulating air in the fridge, so I'll have to add another one to do this (the temp variation between different areas inside this fridge is up to 6 degrees!).
SO, the big test! Pour beer. First few test pours were still rubbish! But the kegs had been out and warming up for a while so I left it until the next day and tried again.
Now, results are similar to what I'm used to. First pour from the tap coughs and splutters and I get half a glass of foam, after that all is good. There does't seem to be much difference between short lines and long.
It would seem that the CO2 in the tap and lines is coming out of solution perhaps? I am thinking that MAYBE I now need to increase the serving pressure a bit, to keep pressure on the liquid in the lines - would that be logical?
When I was having all the foaming issues I had turned the pressure right down. Over the last couple of days I've been increasing the pressure ever so slightly each day (now at about 12psi), and so far no ill-effects.