The gasket holds the 2 parts of the heat ex together.
Half a days work..... 2 hours once you have done one.
Its amazing how many leaky back boilers Ive repaired as most gas engineers quote for a new heat ex, and when I get called in, its a 4 quid part.
The 30 year old Baxi's heat ex's are tough but its a 'niche' to get into... especially when its all iron off the boikler.
Dave
Sorry thought you meant on the combustion side. No nearest I've got to this is replacing gasket rings between section of cast BOA oil boiler - never again.