haza88 said:

Y not stainless out of interest?

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Because stainless is soft, as soft as brass and a lot softer than steel. When you stop suddenly and the seatbelt brackets absorb all that inertial energy the brackets will slice/shear through the bolts like a knife through cheese.
Use (as a minimum) 8.8 or better still 10 or 12.9 HT steel.

miktdish said:

This is the bracket ... https://www.lrdirect.com/adu710050-bracket-seat-belt-anchor
https://www.lrworkshop.com/diagrams...ats/seat-belts/front-seatbelt-anchorage_53637

All held together with M8 nuts and bolts (use 8.8 steel not stainless ...) and some Sikoflex - finished with underseal ...

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noticed yrm sell stainless seat fixing kit bolts , surely they must be strong enough if they can sell them ?

They do, I also use an A4-80 bolt on non critical applications, like holding the seat down, holding the roof on, door hinges etc. But what actually holds the seat down when you are sitting on it in the event of a sudden stop is the seatbelt.
If the seat bolts ( 4 off) suddenly recieved 2400N/mm of force, inside, you'd be soup (think Princess Diana here)... the bolts would pull out of the seatbox well before they failed too.
Also the above is discussing tensile strength, shear strength is different, the normal is to specify 70% of tensile as shear.
Seat is under tension, seatbelt is all shear.

Heres a handy diagram that shows the strength of bolts .... http://www.alliedfastenings.co.nz/site/alliedfasteningsltd/Strength_comparison_steel.pdf
Austenitic 4-80 is just under 8.8 ... 10.9, 11 or 12.9 is what i reccommend you use for the seatbelt fasteners.