Best mastic to seal around a metal flue, and hopefully brickwork too?

I have 2 small sources of leaks on my roof and I'm confused by the array of mastics available.

Can any recommend a repair mastic most suitable to seal around a stainless steel flue which is flashed with lead?

There is also a crack in some rendering nearby letting in water, so if the same mastic would adhere to cement render / brickwork as well, that would be very useful! Otherwise, what would work for that?

Obviously I'm after longevity and ease of application.

Thanks.

Best stuff for lead will also stick to brick, only thing is does it come into contact with a hot flue pipe? Not sure on its heat range a call to the makers is the best idea.
https://www.toolstation.com/lead-ma...MIuN7pmtT45QIVgrHtCh3ZzQd1EAQYASABEgLdAfD_BwE

Thanks for that, I'll check it out. The lead is 'wrapped' around the flue pipe, but I have no idea if it is hot at that level (about 4 or 5m above the gas fire).

It will be twinwall so should be cool enough to use silicone.