no5 john hampden and no12 sarah siddons

That is simple.

They are the same.

IRS use a different convention.

In general terms the locos are BoBo, and specifically 5 = 12 in this respect.

If 5 appears as one and 12 the other in the same IRS book, I suggest that is no more than editorial oversight - and no indication of anything deeper.

IRS often / mostly / always in the EL books document locomotives without visible coupling rods as 'xw' where x = number of wheels in each bogie or frame and 'w' means wheel.

AIUI they do this to standardise records from observations i.e. from what you can actually see - not how it works - or what motors are where which you can not see.

If you take any bogie locomotive on sight, unless you already know, it might really be a BoBo or a BB or 1AA1 or Bo2 or 0-4-4-0 and so on - so they call them all 4w4w; this also avoids other Bo+Bo Bo-Bo B+B B-B B'B' etc etc nomenclature issues where mechanical arrangements differ.

I not seen a recent IRS EL, but in older editions even well known main line thingies like 52s and 55s (Westerns and deltics) were reduced to 6w6w avoiding the same CoCo or CC debate (or e.g. A1AA1A - you can't tell that from sight).

It is just a convention of theirs to try and standardise data, nothing more.

Before someone suggests it, suggesting 5 is 4w4w for no working motors (or any motors) is not the answer; and that answer was given on another forum to a similar question about a different loco type some while back. IRS would still list it as 4w4w irrespective of how many motors in place or capable of working.