Well, at least: SOLVED.
Stainless steel but not the very thick ones... 10 pounds less than at MGF TF bitz...
Then, threatened that 14mm bolts could break... I've been looking for 12.9 Allen bolts, instead of Rimmers standard ones... came upon a couple of Ebay sellers not answering or not shipping abroad... = £4 each.
Bought luckily + by chance from a french provider (MAVISSERIEAU DETAIL) 2 pairs 10 + 12 cm (didn't remember the right sizes of front + rear mounts).. + 4 Nylstop/nylock bolts. =====> 12€
Anyhow, after 20 mn bolt unscrewing each with spanner extension , none broke.
Never had such a physical work for 2 bolts
Here is the end!