Where best to fit an SPD all depends on where the surge is generated, and where the victim is, and the type of damage mechanism the victim is likely to experience.
The simplest  is when the impulse is external to the building, and is common mode relative to ground - that is the voltages on  L and N fly up together- rather than voltages moving   in differnent directions - that antiphase  case  is differential mode..

In such a case an SPD near where power enters the building is enough. But, surges may come from other places - wiring to outbuildings ,workshops with welding and machines, and may be a mixture of common and differential modes.

In a perfect world each needs strangling as near the origin as possible - but this is not always practical. Situations where the victim equipment straddles two zones - perhaps mains wiring and a phone line or cable tv, then it may only be possible to tie things together  the victim to avoid the electrical equivalent of the splits...

there is no one size fits all solution that always works.

Mike

This ought to be a simple question!

If there is a main DB at the origin with distribution circuits to different parts of a house and its outbuildings, there seems to be nothing at risk before the final DBs.

If, for example, it is desired to replace the final DBs in order to provide additional protection in the form of RCDs, doesn't it make sense to put the SPDs in the final DBs, particularly now that DBs with SPDs already installed are widely available?

Does it indeed make sense to put SPDs at both the main board and all dist boards both local and intermediate? perhaps it does but the cost versus risk needs be analysed too. In fact do you all use an SPD trailing socket to plug in computer stuff to the nearest socket outlet as well?

Chris Pearson said:

This ought to be a simple question!

Yes!

I make an intelligent guess that only 1% of all houses in Britain have a SPD in the incoming supply.

There are still plenty of houses with prehistoric consumer units fitted with rewirable fuses. Even more with plastic / bakelite consumer units where the original fuses have been replaced by MCBs.

Chris Pearson said:

This ought to be a simple question!

Yes!

I make an intelligent guess that only 1% of all houses in Britain have a SPD in the incoming supply.

There are still plenty of houses with prehistoric consumer units fitted with rewirable fuses. Even more with plastic / bakelite consumer units where the original fuses have been replaced by MCBs.