Hi.  It's great to see there is still a thriving EP group!!!

Hi! Welcome here!

My latest check reveals that all is not well - it will not power-on at all...

I have also been able to deduce that all supply voltages are present, (+5v, +12v)

Do you checked both +5V voltage?
The machine is always used with the correct PSU? (The wrong polarity can be damage the machine...)

which is being supplied to the Z80.  However, there appears to be no address/data bus activity at all

I think very possible the Z80 is deaded. In about the the last two years I found a five dead Z80 cpus. Four in Enterprises and one in a ZX81. The machines come from different users, but with a very similar story: machine worked at many years ago, not switched on for many years... And now it is deaded...
There is the first three deaded Z80 cpus.

nor is there any RF (PAL) being produced by the modulator (no raster of any kind).

Can you check it via RGB Scart cable? Independently the cpu, the Nick chip (the video chip) is generate a random signals on the screen depending on the randomly filled videomemory and registers at the powerup. Normaly you not see these, because the ROM program quickly initialise the video chip. But if the ROM or the Z80 is deaded, then the video chip is not initialised, you see random signals on the screen! Some times it is stay black due the randomly initialised registers, but after few powerups you always see anything on the screen. The most typical is a running thin lines.
But if possible some TVs not understood these "garbage" signals, and not display it.

Do you checked the Z80 RESET signal? And the HALT, WAIT, etc pins, it is not stoped with wrong signals? If everything ok, I think firstly lets try to replace the Z80.

And some help: collection of Enterprise schematics.

Do you tried to see the RAS/CAS lines of the video RAM ICs? It is also no acitvity?

Maybe I was wrong about no activity on the video connector - the pic below is the best signal I get 'top-side', while probing the bottom connectors, I get random spikes of about 200-300mV!!!  Also, both RAS/CAS of video ram are active which is nice to see   Pressing the reset button results in no activity on the Z80 reset pin, so maybe DAVE is the problem!  C4 is also fine.  At least some activity now, which is nice to see...

PS - I just realised that Z80 RESET is always low, and pressing the button doesn't effect this either - which doesn't seem right...

Zozosoft, hello again!  There's good news and bad news - as expected, the no address/data bus activity was caused by the Z80 RESET line being held low, so once this pin was disconnected from DAVE and wired directly to the reset button, I then had bus activity restored.  Unfortunately, as you pointed out, DAVE also supplies at least one other crucial signal, the rom's chip-enable (CE), and this is always held high, so the rom is never activated!  As a result, the video side cannot be activated either, so no video as well   The U37 inverter associated with DAVE's Reset line is fine, and DAVE does not appear to be dry-jointed.  So it certainly looks like the DAVE i.c. is the culprit.   

the 4.433619Mhz, 8Mhz and the derived 4Mhz

The 14Mhz which is used by the Nick, is present? And the 1Mhz from the Dave?

Looks like I will just have to wait until someone decides to sell a faulty Enterprise and hope the Dave chip is OK!

Do you can replace the surface mounted Dave chip?
I will search a "donor" for you!

something as 'big' as DAVE/NICK would be no problem at all!

It's great!

If you knew someone with one for sale, it would be fantastic!!!

Ok, I start searching!