Normally I am a big fan of the LSD technology like many others here. I bought the accupower evolution D cells. I don't know if you are aware of the LSD or hybrid batteries? Essentially you give up a little capacity in exchange for a low self discharge rate meaning that you can leave them sitting around and when you get back to them they will still be charged. I am sure you could find a discharge graph somewhere here, but should be along the lines of only losing 15% after 12 months. It's linear though, so they lose the most in the first month and slows down after that.
In your case you may want the maximum runtime and not be worried about self discharge. Some comparisons have been done, the more traditional nimh cells can vary a lot. Some will run down in only a few weeks and some can go several months and still have more capacity than the LSD cells. This is affected by both the brand and the age and condition of the cell and just how it has been treated.
If it was me, I would be more interested in what I could buy in the UK even if it wasn't the very best. I mean sure, you might get a battery that is 10 or 15% better at a huge premium if you had to have it shipped from US, but I would be interested in a cost to performance ratio. Perhaps something from Ebay UK? Someone has probably written a thread about what is availible to you locally.
Stay away from energizer, duracell, etc in this case, as they are just selling a AA battery in a D package, a 2500 mAh D cell. Yuck.
Speaking of, another option is to run AA's. If you haven't seen them they sell some pretty cheap adapters you slip an AA battery in and fit them in a D size holder. One comes to mind that holds 2 AA's side by side, doubling your capacity. So in your case you could run 4 AA nimh batteries in a 2x2 configuration. 2.4V at 4000 - 5000 mAh. The cheap ones aren't good for higher current applications, but yours isn't that high a current in the grand scheme of things. The only concern there is it seems like I remember there was a contact problem either at the top or the bottom of the light. So you might have to do a slight modification to either your top spring or bottom spring of your mag.
Do you happen to already have an AA charger and or batteries?