Whilst I have not used Cementone myself I have several builder associates that swear by it. Brush it in, job done, couldn't be easier. Not sure on the longer term quality as I've only really known of it for a year or so.
Downside for you is that getting a colour match on a part job will be tricky, even though it does come in three colours. The other issue is cost (for a small area), as once opened it has to be used. You can get 20Kg for £29.99 at Screwfix last time I checked.
One last point worth noting is that we explain the risks when cleaning such paved areas. Even when taking as much care as possible there is always a risk of blowing loose compo. If this is made clear from the outset and obvious problem areas identified then there can be no comebacks. We have this included in our T & C's.
As a back up, and at an additional cost to the customer we do have a company that we recommend for repairs / refurb's etc. We used to add a commision but we get work from them and vice versa, it also helps win the trickier jobs.
In this case I would probably opt for pointing with self-mixed compo, unless you are sure of a colour match.