For those that think my question is too general (hillrider in particular) or have the energy to pursue this further, search .... sample dates 13 Feb out 20 Feb return.

For the outbound you will see the lowest business class fare shown is £2,709 (approx) and this will show (if you click the + sign on the left) as I/I. Pick as the return, any of the First fares with a £36 supplement. These correctly show as D/P (this is genuine first class P not an upgraded economy fare).

The booking page will show a total fare of £2,744.75 (or thereabouts depending on FX rates). The trip details will continue to show I / I / D / P as at the earlier stage.

However, if you now pop up the fare rules you will see that the 2nd leg is not showing as I but rather L with an LNE0C0Z1 fare code. If you look at the rules for both the first and second legs they show as being identical to the P class fare on the return, with a cancel / change £200 fee (in the Penalties box).

The rules are exactly right for the P class fare but, if you then look up both the I4UKJB and the LNE0C0Z1 fare codes on Expertflyer they both show as completely inflexible fares. In other words, aa.com is applying not the most but the LEAST restrictive set of rules to the whole journey.

Hence the problem. if an agent simply looks up the fare rules in the book they will show as inflexible, but if he accesses what aa.com shows on the booking it will show as semi-flexible.

I would repeat that I have resolved the problem for the specific trip but it does leave me wondering if aa.com is reliable.