I tried the out-of-the-case troubleshooting technique today; the result is the same as before: spinning CPU fans, no beep, no screen signal. One suggestion that has been made to me is to try fitting a new CMOS battery; if the CMOS battery were empty or broken, could it cause these symptoms?
If not I understand that the next step is to start looking at individual components. I assume my RAM is OK (tried different sticks), my CPU too (it warms up). This leaves the motherboard and the graphics card. Common sense tells me that a broken graphics card should not prohibit a start-up beep, meaning that at least the motherboard is broken, (and possibly also the graphics card). Correct?
With the broken HD, this leaves little of my system. Apart from an empty CMOS battery coinciding with a HD failure, I see no other explanation than that some kind of power surge broke it, and am therefore considering building a new PC, unless anybody has any further suggestions of course..
Thanks again for the support; though the final result is not very satisfying I do believe I've gotten more or less to the bottom of this.