I had a look at my own router status page, and it shows ~18meg D/L and I normally get about 12-13meg on speedtest.net. Hence my concern at the huge discrepancy my friend is getting. He's about a mile and a half from the exchange, and the usual ~3.5meg speed is fine for his purposes. Yes, I forgot to try a different filter, but figured the reported up/down speeds would indicate this wasn't an issue. I'll take a known good one with me and do a quick swap first.
IIRC, on the previous occasion the Openreach "line engineer" didn't seem able to hear the quite obvious "hum" that signalled to me an earth fault, and I had to be quite insistent that he looked further. The first span of dropwire was going through a tree FFS - the very first place I would have looked...