Had a similar experience with other cars before and it sounds like a tie rod (I think that was the
problem I had) might be at fault. The giveaway comment was the knocking occurs on rough surfaces. Does the knocking go away on smooth road? (rare I know!!

)
I'll be honest: I heard symptoms (load knocking) on Tuesday, then when I tested it on Wednesday it wouldn't bloody replicate! Just my luck.
The noise was like driving over gravel, with a large rock smashing into the wheel well.
The AA man (cost covered by SEAT Roadside Assistance) said that the braking noise is not normal, so he'll follow me down to the garage. The weird thing is, SEAT Roadside Assistance told me to take it down to the local VW, who upon arrival were utterly confused as to why they had it, when there was a SEAT garage down the road (the reason is, they have a body
shop, SEAT don't - apparently). I was left in limbo because they (SEAT Roadside Assistance and VW) obviously hadn't communicated with one another. It was a bloody farce.
I've asked them to jack it up, check over everything properly and if there's nowt' wrong (there is, and I won't accept that decision if they say so) to charge me for the labour of checking.