SNG Barratt have their own stocks of a lot of parts - they are always worth checking if Jaguar Classic parts don't have what you want anymore.
I have frequently found SNG have stocks of parts no longer available at JCP, or have sourced something similar. If SNG Barratt source the part directly from JCP, they will mark the part with a blue 'Special order' sign; if the part is in stock, they will mark it with a green 'In stock' sign. When they mark something as 'In stock', it means they have the item in stock, they won't be dependent on JCP to supply it.
Likewise, David manners hold stocks of items no longer available from JCP - though I have found them less frequently able to supply parts no longer available from JCP than SNG Barratt. If David Manners are sourcing the part from JCP and don't hold any themselves, they mark its availability as '2 days'. If they hold a stock, they mark the part as 'in stock'.
For David Manners, most of the in stock parts seem to be aftermarket ones. SNG Barratt however seem to hold quite a lot of genuine parts in stock.