It is not as simple as if they instruct your quote to proceed then by default your quote is agreed. A clause 61.2 is a “what-if” scenario. If I was to ask you to do something, what would the time/cost implications be? Once they have the quote, they can then decide whether they want to instruct it. It is not helpful the words that they have added i.e. “cost to be agreed” but instructing under 61.1 should be clear that they now wish you to proceed. They should not need a new quote as they have yours, but it does not stop them now confirming if they don’t agree with your quote, at which point they could either ask you to re-quote or make their own assessment if they don’t agree. Either way this should be brought to ahead within the timescales of the contract. It is in neither Parties interest to leave this un-agreed and then have a subjective argument about actual cost some point later in time.
I do think the contract could do with slightly better wording on what happens to a 61.2 quote once it is submitted i.e. how that quote gets instructed to proceed and how the quote is assessed.