So with the memory mapping on, the torture test didn't hit any errors. 
So I'm tempted to use this as a short-term solution, and then at the 
weekend I'll looking at swapping the sticks around.
For anyone that's interested: you can do this (on debian) by editing 
/etc/default/grub and to the line that begins:
GRUB_CMDLINE_LINUX_DEFAULT=
add something like:
memmap=100M\\\$800M
which will tell the kernel not to use the 100MB of memory starting at 800MB.
Leo
On 17/05/15 12:13, Neil Stone wrote:
> Before you do... swap them 1 for 1... you can possibly identify which
> stick is faulty...
>
> On 17 May 2015 12:09, "Leo" <linux@???
> <mailto:linux@fractal.me.uk>> wrote:
>
>     Hmmm, hadn't thought of that either. There's four slots and two
>     sticks. So I could move the two sticks to the free slots. I'm
>     running another mprime test at the mo, so I'll give that a go later.
>
>     Thanks,
>     Leo
>
>     On 17/05/15 12:06, Neil Stone wrote:
>
>         What happens if you swap the ram slots (assume you have more
>         than one
>         ram stick) ?
>
>         On 17 May 2015 12:02, "Leo" <linux@???
>         <mailto:linux@fractal.me.uk>
>         <mailto:linux@fractal.me.uk <mailto:linux@fractal.me.uk>>> wrote:
>
>              Unfortunately none of my computers share the same RAM type
>         so I'd
>              have to purchase some more for that. So I thought I'd give this
>              memory mapping a go first.
>
>              Leo
>
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