The thing to keep in mind about the seals is a (kebab) WB3 seal is mainly a premix air/gas seal, not sealing combustion other than around the electrodes. Pre mix air/gas leaks don't need much thinking about other than don't let them happen. The seal on the CDi clasic is a combustion seal so I think you can apply a bit of on site common here.
The seal on the clasic that needs carefull attention is the electrode seals, like the kebab seal never re use it. On kebab heat exchangers I now replace electrodes and therefore seals as a matter of routine every five years due to the increased erosion of the ionisation electrode. This has got worse over the last few years, right or wrong I put this down to the increase in sulphur in mains gas.
This obviously has an impact on cost but if you are servicing as per MIs you have four years of tring to look honest, hard working and professional before you ever get a spanner out the bag.
If you have a concern about a kebab seal for instance you have a new customer, with an older boiler use your FGA to check for hot spots around the clamp plate and electrodes.