You simply cannot use provisional sums in an unamended ECC contract. They only work in other contracts (such as the JCT) because the contract explains how they are to be used and how they change the BOQ. The ECC has no such provisions. So the answer to your question will lie in the (carefully written) Z clause that you included in the contract setting out how these provisional sums were to be used. And if you did not include such a Z clause then you have a real problem.
The problem is that the removal of this Provisional Sum may (see next paragraph) be a compensation event under 60.6, but what effect has it had on defined Cost? It has not changed the WI, which, I assume showed no asbestos removal (remember the BoQ is not WI – see 55.1). So what should the Contractor have allowed for in the Provisional Sum? The answer is probably nothing, because you have not asked him to do anything. Since the compensation event is assessed as the effect it has on defined Cost, it could be argued that it has had no effect, because nothing was required! Your suggestion that you assess the Defined Cost (using the SSCC) of what was supposed to have been allowed for in the provisional sum is impractical because you do not know what you wanted to do (that is why you made it a provisional sum in the first place) so how can you calculate what it would have cost to do something you could not define?
On the other hand it could also be argued that this removal may not be a compensation event at all, because the BoQ showed something that was simply not required, so it was not a “mistake” as such as defined by clause 60.6, because it is not a departure from the rules etc. in the method of measurement, or due to an ambiguity or inconsistency. Instead under the definition of PWDD (11.2(28)) this work is never completed so is never paid for.
Which of these competing arguments (or any of the many others I could think of) apply is a legal answer that the helpline cannot answer I am afraid.
Please do not use provisional sums with the ECC. If you do not know what you want or are not sure if you want anything, then say so in the WI and say that instructions will be issued if and when you do know. Such instructions will be compensation events if issued, but will not be and will have no effect if, as here, they are not. It will be assumed that the contractor has not allowed anything for these in either his price or his programme. That, by the way, is exactly how undefined provisional sums are dealt with in the JCT contract.