The ascorbate 'reduces' the nitrite giving added protection against any potential cancer forming nitrosamines that may affect people years down the line. It can be omitted; particularly for people who have already reached a grand old age.
Using ascorbic acid which is also a type of vitamin C has been discussed before and I think the verdict was that it's OK but has a different 'molecular weight' so is used in different proportions; if I remember correctly it's used at 469 PPM. Zulululu's post about the nature of tablets is worth noting though.
There are sources for ascorbate online:
<a href="http://www.homeherbs.co.uk/0/product/0/562-Vitamin_C.html" target="_blank">http://www.homeherbs.co.uk/0/product/0/562-Vitamin_C.html</a>
<a href="http://www.healthplus.co.uk/pages/sodiumascorbatepowder.htm" target="_blank">http://www.healthplus.co.uk/pages/sodiumascorbatepowder.htm</a>
<a href="http://www.healthinajiffy.co.uk/health-plus-sodium-ascorbate-powder-v250g-p-55.html?currency=GBP" target="_blank">http://www.healthinajiffy.co.uk/health-plus-sodium-ascorbate-powder-v250g-p-55.html?currency=GBP</a>
As the above cure reduces the levels to allow for 10% skin on the meat, this would mean using it 422 PPM (469 - 10%) or 0.42gm per kg in the recipe above, rather than the 0.5gm ascorbate.
Dextrose is the food industry name for what is usually sold as glucose (it's actually Dextrose Monohydrate), you can buy it in powdered form from a good chemists shop (ask for glucose powder). It's a type of sugar that's less sweet than 'table sugar', it's used sometimes in fresh sausage so that they brown-up nicely when you cook them and it's also used as a reserve of food for the 'starters' used in dry cured sausage. Why you would use it in a brine (instead of just using sugar), I don't know.
Phil
edited to add sources of ascorbate 19.50 18-May-09
