Hi MJB and welcome.

Just to reinforce Amanda's question - as you say, the view into the enclosures is a facinating - and obvious - speculation if Silbaby was there at the time, yet neither Pollard nor Whittle mention Silbaby at all, despite spending so much time right next to it.
The obvious assumption is that they knew exactly what it was, and that it was an irrelevance, and we're barking up the wrong tree. Yet we still can't seem to find anything that indicates what it is.

Yes, lots of archaeologists and antiquarians, and others of course, have trampled around the general vicinity over the years without noticing Silbaby. It may be that they saw it and didn't think it was of interest (ie not prehistoric), but I suspect in most cases it wasn't seen. This is not an isolated case - even very distinctive monuments such as long barrows can still be found lurking unnoticed in the landscape. A major part of the problem with Silbaby is visibility, especially during summer when vegetation (and visitors) are at their greatest - it just isn't very easy to spot. Now it's been spotted, it's difficult to believe no-one had noticed it before, but it really does take someone like Pete to put on a pair of wellies and go looking. An underlying factor may be the watercourse - channels get tidied and straightened, water meadows created etc - it is unusual to discover something of prehistoric date still extant and that close to water, which may also argue against its antiquity.
Silbaby is only partially visible on aerial photographs, and there are limits to how low we can fly. As for what Silbaby is - clearly some landscaping has been going on, but as far as current knowledge goes, it's really anyone's guess. Personally I don't think we should rule out a relatively recent date - right next to it, the river has been artifically channelled and a lake with central island formed, so for example we might be looking at some landscaping associated with the nearby Manor House, perhaps with some enhancement of the view of Silbury as part of it. But I could be completely wrong of course (and I can provide you with any number of examples of me being wrong!). Anyway, other colleagues (including the previously mentioned Dave Field) are taking a closer interest, so I'll wait and see what they (or indeed anyone else) comes up with. But please remember - the EH survey staff are small in number and, tipping aside, Silbaby itself isn't under any real threat - unlike the palisaded enclosures, it isn't being ploughed every year, for example - so other sites tend to take priority at the moment.