samuria said:
It will remain with new router if the rest is set to dhcpit works best for all devices
So I haven't yet played around with changing DNS but I wanted to update you since you helped me. I bought a new network card which arrived the same day as the virgin media installation. On the new 100MBPs virgin media connection I got 110 MBPs and....all the problems were completely solved (with the original built in network card)!

When I changed back to the old origin broadband connection I disabled the normal network adapter in device manager and installed the new one, and that also solved all the problems. The speed is now constant and doesn't jump up and down, the buffering is (mostly) gone and downloads are back to normal (original) fast speeds. I think the fact that some buffering does still occur (3 times in 15 min on Iplayer) reflects the only thing which hasn't changed: the max speed is still coming up as 8.9 MBPs (but at least it's now consistent).

So the big question: what actually caused all this? With the original NIC when I plug in the virgin ethernet the problems are gone, so at 1st I thought the culprit must be the Openreach connection (probably within my premises). Then the new NIC solved the problem also so I'm back to thinking it could still be the old NIC which somehow developed a fault (taking into consideration the fact that all other devices were always working fine)?

Before we had the torrential down-pour of rain that day, we used to get around 12 MBPs and immediately after that day and all the problems it brought (re-read the OP) it was 9 MBPs max and completely unstable. Now it's still 9, but stable. The problem is when my current Virgin contract expires the price will go way up and I'll have to go back to an openreach connection, so I really do want to try to figure out the solution. But Origin broadband (and any other company I'm guessing) say if they send out an openreach engineer and their equipment shows my PC to be at fault then I'll be charged £175 or whatever it was!