Quick update on this... I spoke to Bob Rawle on a slightly different subject as I am going to fit a TD04 which will then require a remap. Bob, being Bob, is in huge demand and won't be able to remap my car until the end of July:icon_frown:
As I'm not sure when I'll be able to fit the turbo - due to wife and children, I may need to fit it some time before. In order to prevent overboost from the larger turbo, Bob suggested connecting the turbo outlet directly to the wastegate actuator, limiting the boost to about 0.6 bar according to Bob.
See the diagram on
Boost control system
In other words, connecting Line 1 and Line 2 in the diagram together without the T piece in the middle.
I had a think about this and it is, in effect, this is doing the same thing as Simon's fix... the only difference is that Simon blocks the pipe after the boost control solenoid (BCS). Both methods prevent the BCS (controlled by the ECU) from bleeding pressure away from the wastegate actuator and thus the wastegate opens on it's spring pressure (c. 0.6 bar).
So, my theory is to get a 2nd BCS and fit it after the 1st BCS. The 1st BCS will still be controlled by the ECU and will open and close to as per normal to control boost but the new, 2nd BCS will be controlled via a switch so that it will either be open (leaving the 1st BCS to control the wastegate) or closed (preventing the 1st BCS from venting pressure and therefore the wastegate will open at it's default spring rate).
First off, I'm going to try Simon's method and monitor how the engine copes with just running on wastegate pressure (I now have a ODBII cable and ECU logging software! :woohoo

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In theory, the ECU should read the manifold pressure (@ 0.6 bar boost) and then fuel/advance ignition accordingly with no big drama. If this goes OK, then I'll attempt the switchable version!
Will let you know how I get on.