Everbuild Everflex 500 Bath and Sanitary Silicone Ivory Anti-Fungal Sealant 295ml £3.95 @ Amazon

Decent sealer. Dow Corning is usually my go to. Blackened or failed silicon a problem? Then take time to dissolve old silicon by getting a pot of silicon remover. Clean it off with some wipes...allow to dry. Mask the area to be re-sealed allowing 3mm above and below the area/gap then apply the sealant. Run a wet finger along the area and remove excess on a cloth and continue until you've competed the whole area being sealed. Then remove the masking by gently pulling away and dispose carefully. Should have a decent sharp finish but if not gently run a wet finger again just to rid of sharp / jagged seal (usually it will be ok and not needed). You'll be proud of your efforts! Condensation on walls and ceilings? Remove with white vinegar, allow to dry and apply proper bathroom paint. Mask tiles and woodwork, upvc window frames etc for a clean sharp finish. Periodically microcloth the walls and ceilings for longevity. Opening windows is key to reducing mould etc even in winter. Hope this helps anyone. Bit of a soapbox session but this platform is excellent to help the consumer!

Get a Cramer Fugi Profiling Tool and get a better finish, you can also touch up on the go because you don't need to use a wet finger or masking tape.

Just what I needed to replace some black moulded silicone around my sink.