RSOL,
The officiallength of St. Mawgan's runway is 9006', this is the usable length available for 'legal' purposes.
Size of the aircraft is irrelevant, I previously worked with the AN124's and it is known that they have a good field performance for their size.
I don't have the performance figures for every type of aircraft, the DC8 was merely an example, I would guess that the 747-400 would be OK but what about 747-200/300? I don't have these figures, perhaps they may be OK but then perhaps not, but there are other 'heavy' aircraft cargo types out there, of lesser performances that more modern aircraft, but St Mawgan's published runway length of 9006' is not capable of handling any aircraft type flying today which is what you stated before, more recently, changing your previous statement to include the word "nearly" which hasn't gone unoticed!
St. Mawgan has perhaps the largest surface area of any runway in the UK today, taking in to consideration it's width, and the chances of anything heavy, on a regular basis, using it are pipe dreams! There's little of a catchment area and with less than ideal road connections from/to the Cornish peninsula particularly during the summers.
And I was working for the NQY airport operator (lease holder) as far back as the days when an HP7 aircraft operated the LHR route.