NEVER use mild steel bolts! That would be very dangerous. The ONLY acceptable bolts, and what would have been in your car already, are high tensile grade 8.8, or if you want to waste money, or happen to have some already, or use capheads, grade 10.9. Grade 12.9 should not be used on account of it being more brittle. (I recall that only 8.8 and 10.9 are allowed on seatbelt bolts for that reason). You may be surprised to learn that car bolts are a higher grade of steel than those that hold most bridges and other structures together, although civil engineering has finally caught up with "high strength friction grip" bolts, which are actually only about grade 8.8 but with special heads and washers.
If you need the taper ended bolts I would try somewhere reputable like Somerford Mini, or apply the taper yourself, carefully, with whatever tools you have. A lathe is ideal but you can carefully file or grind a reasonable taper by hand. Put a nut on first, when you remove it, it will clean up any minor damage that you have done. Needless to say, the taper must only be beyond the captive nut, which must be fully engaging unmolested bolt thread.
If ANY of your bolts do not show 8.8 (or if very old, S, the old British Standard grade, basically identical to 8.8) on the head, and if nuts are not marked for grade 8 or above, they must be changed. If you take to the road with mild steel fixings, a serious accident is likely. Note that B&Q sell only mild steel bolts, they must not be used. Oh, and no plated bolts, such as chrome or nickel, ever, unless they are supplied with paperwork showing that they have had the proper post-plating heat treatment to avoid hydrogen embrittlement, which is very dangerous. Mechanically zinc plated bolts (sherardised) or hot dip plated (galvanised) are ok, but electroplated zinc most certainly is not, without the heat treatment. DIY plating kits should carry a warning, but usually don't.
See below for info on bolt markings, but note that metric (ISO) markings e.g. 8.8 are frequently used on imperial (5/16" UNF in this case) bolts and that is ok. Also US Grade 5 is actually 8.8. US grade 2 is mild steel. Don't use ISO grade 4.6 which is mild steel.
https://www.boltdepo...rade-chart.aspx
http://www.thomsonra...s and bolts.pdf
The second link shows the correct head and nut markings. Never mind it referring to metric bolts, which are not used on most Minis, the same markings are used on UNF bolts nowadays.
I am saying all this because it is important to ask for what you need as far as bolts are concerned. The supplier can't usually guess what you are doing with them. In most cases just order up the appropriate length, diameter and thread, usually UNF, clearly specify grade 8.8, and you will get the right thing. Otherwise you will usually get mild steel....
Exhaust bracket bolts are usually of cheap galvanised mild steel, because they are not very highly stressed and get replaced regularly along with the exhaust, so please don't procure anything except exhaust system bolts from Kwikfit or similar. It is perfectly ok, if you have a stock of 8.8 bolts, to use them on your exhaust of course.
