I haven't a clue where this was copied from but I feel that it was from this site. Mmmm, there's a problem. In the book I have made it more clear than on the website but the recipes DO NOT scale up well beyond being doubled. That is because they are not scaled down versions of restaurant procedures but rather a method I devised to make domestic sized portions.
If you are making curries AND vegetable dishes then you WILL need the equivalent of 10 times the amount of basic curry sauce (BCS) to make a meal for 20.
Scaling up the curry recipes that much will NOT work either. What will happen is that you will get a kind of curried stew rather than the stir-fried effect of restaurant style curries. Also the spicing will be all wrong. You would certainly need to reduce the stronger spices, especially chilli powder, a fair amount. Getting the curries too wet will be a big problem and water should be added in very small amounts.
So, what to do?
You could hunt out some more traditional recipes where the curries are cooked for a long period of time and in larger quantities but that will not produce restaurant style curries which is what I expect you are after. Even a restaurant would probably make 10 x double portions of curry if you turned up and ordered the same curry for 20 (not the BCS, of course, that would have already been made in bulk earlier on in the day).
If you are committed to making restaurant-style curries then I can only suggest what I would do in the circumstances although, as you rightly point out, I avoid doing it like the plague and have never stretched to 20 (12 people is about my limit)!

Thank you both, I'll hunt some down

You are lucky to have found tandoori paste as I can't find it anywhere, I believe that I saw on here previously that it had disappeared from the shelves and have looked everywhere locally, which shop did you buy it from please?

Unfortunately I have no room in the freezer it's full of base sauce and pre cooked chicken as this does seem the perfect solution. I do have other things in too but this website is so good I'm starting to get carried away lol!

Great idea, taking up much less room than the plastic containers in the freezer

Very impressive for a first try and I must say your tidy array of spices are something that I'd love to achieve one day, mine are stuffed in a double cupboard, some in packets, some in jars, some in boxes and take an eternity to find before I even start to cook. Reminder to myself: Must get more organised!

I found another one

Garp's spice mix

3 Tbsp Cumin
3 Tbsp Coriander
2 Tbsp Turmeric
2 Tbsp Paprika
1 1/2 Tbsp Fenugreek
1 Tbsp Garam Masala
1 tsp Fennel

When I came home from India earlier on this year I brought quite a few spices with me thinking that they would be the real 'McCoy' Yesterday I opened  a silver foil packet of methi to make some shami kebabs and later on after I had added the dried leaves to the mix noticed tiny little insects crawling on my cook book and on closer observation found others in the bag too, so there goes 1 klo of minced meat and 500 ml of chana dhal into the rubbish bin and lesson learnt, no more spice from India when I next go. Anyone else noticed these in home bought packets?

I used unsalted butter when I cooked this recipe as I often find that there is usually too much salt already added to a lot of curries on here for my taste, as I said before, nothing  wrong with this recipe, it was excellent

I cooked a batch of this curry base sauce today and must admit that it's the best one I've tried so far, I added Cory Ander's spice mix as I had some already left over. The taste is superb, in fact I could eat it just as a curry sauce, thank you JB so much for this.