I've asked the question previously, on the place where people typically mount the relay when modding the fuel pump wiring, but haven't been given an answer, so I'll discuss my intentions and see what responce I get. I assume from the "how to guides" the relay is normally placed near the fuel pump. I've got a couple of concerns with this, 1. if its placed under the boot floor, any petrol vapours that could be around from a leak could be ignited from a spark from the relay , and 2. as suggesed in the guide if the inertia trip is activated there is still a live feed to the relay.
My intention is to mount the new relay under the dash on the drivers side and break into the swiched side of the inertia switch for the new relay connection to terminal 85. The remainder of the connections would be as previously suggested. This option seems both neater and safer, but I am open to comments if I've overlooked something.

Not sure how a spark in the sealed relay would ignite any vapours... I've had mine sitting om the floor for years, but for a belt & braces installation, you can do what you propose, but that defeats the object of the live feed, which is to get as high a voltage as possible to the pump. Go on, live dangerously!

I thought a relay would be a sealed unit, so shouldn't be able to generate a spark if petrol is in it. Having said that, my car smells of petrol all the time (due to swirl pot, another fuel pump etc..) and I haven't ignited the air with the relay in the boot.

Also, I have the relay and it turns off as it should, so there shouldn't be any reason to tap into the intertia switch.

Couple of points. Standard relay will not be intrinsically safe, ie not completely sealed, so fumes could get into the relay and then be ignited. Second point is the reason for the mod is to provide a low resistance feed to the pump so as to reduce the voltage drop. My proposal is exactly the same as what has ben done before other than moving the relay away from the petrol tank area, reducing the risks.

The original fuel pump takes power from the interia switch, and the wiring modification is to divert that power to the relay, with a permanent live direct from the battery, and very short wires from the relay to provide the (inertia switch) switched live and a short earth cable to the boot earth point.

If you place the relay outside the boot area, you'll need a shorter direct live from the battery to the relay, and a longer switched live from the relay to the pump. But the relay can go wherever you like. Lazy boys like me and Beggers have put it in the boot, sounds like you will put it somewhere else...