A while back there were sill panels available from a club member but I don't think a new batch has been made so you will probably have to get some sheet metal and get folding. Some restorers have taken the body skin off the substructure to carry out repairs but its a major undertaking and often not necessary.
The link below should give you a good idea of how the repairs can be tackled which is very much what we did on my 401. When the time comes BODA will be able to supply the screen, door, side window and body to bumper rubbers, also reproduction badges and lots of other bits. Also speak to Graeme Payne at CBCPC to check what he can supply.
https://lightscarsaction.com/bristol...l-restoration/
P.S. Just checked and this panel is still available on ebay, the seller has other bits and pieces for the 401/403 that could be useful.
https://www.ebay.co.uk/itm/134347409...gAAOSwQfFjh2kE