I think it'd be really unlikely anyone would get done just for trespass, as it's a civil offence, not a criminal one, so the onus is on the landowner to press a civil case. Usually this wouldn't be worth the risk unless it's a persistent repeated trespass and the wonderfully titled 'Injunctive relief' is sought. So if some crazy type were to repeatedly climb Silbury on a regular basis, there would be legal methods to lay out an injunction.

But I reckon EH could get somewhere with it on the grounds of potential damage to a SAM. The EH Silbury signs do indicate that climbing might cause damage, so in a far-fetched example, anyone trespassing with a metal detector would be fair game to be charged with an actual criminal offence rather than just trespass.