It's not difficult to malt wheat and make your own malt flower, your chances are pretty good that if you find organic wheat berries in the store you'll be able to sprout them (for malt you want the berry fully utilized, which is about when the sprout is the same length as the wheat berry itself.)  Then all you have to do is dry it in the oven at a low setting of no more than 170F to keep the enzymes active and grind it like you would any other grain... assuming that you have a mill.  Or use a coffee grinder and make a coarse flower and a rustic malted WW loaf.

Last time I bought malt one of the varieties came from British company called Simpsons Malt. 

The brewer's malt might not be a bad route to follow.  Here in the US there are homebrew stores all over the place, each carrying several different varieties (including kilned and roasted as you mentioned).  True, you would need a mill, but you could have the homebrew store crack it for you in their roller mill and finish it off with a coffee grinder, as nguy78 suggested.  I probably have 20 lbs of malted wheat in my basement that I'm never going to use, I was on a weissbier kick last year.

You might try http://www.heygates.co.uk/heygatesuk-mills.html, I know they do a wonderful malt crunch flour.

In the UK there are 3 suppliers that I know of. Their flours are available in health and organic food shops and direct from the mills (well worth visiting)

http://www.bacheldremill.co.uk/MALTED-BLEND-HR-200.jpg 

http://www.fwpmatthews.co.uk/cotswold_flours.php 

http://www.shipton-mill.com/301-Organic-Light-Malthouse-Flour.html

Thank you all for your ideas. Sadly all the suggested UK mills seem only sell blended flours containing malt or malted grains. What I'm looking for is a pure malt flour, which only seems to be available in bulk quantities for commercial bakeries.

As for milling my own flour, that would require me to purchase a mill. There is one available for my Kenwood Chef mixer, but I can't justify the cost just for this one use. I'd never get it past the finance comitee! There isn't much tradition of home milling in the UK and so supplies are not too readily available, although I agree that brewing malt can be obtained relatively easily.

For now, I think I will have to stick with the malt loaf mix from Wessex Mills. I've now found this in a farm shop in nearby Abingdon, which is nearere than going to the mill which itself is only 10 miles away.

I ordered diastatic malt flour from the American Spice Company (www.americanspice.com). It arrived in 2 days.

I emailed the UK Flour Advisory Bureau, explaining what I wanted and remarking that Muntons had not replied to my email. Their helpful reply was as follows:-

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Try and contact the Muntons Malted Ingredients - tel -01449 618 300
And also make contact with the following:

Edme Ltd - tel 01206 393 725
Rank Hovis Ltd - 01427 613 502
Bacheldre Watermill - tel 01588 620 489

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Note all these telephone numbers are UK natioinal numbers.

I had already found 3 of these company's web sites but hadn't tried phoning any of them.

I will try these over the next day or so and report back.