<font color='#0000FF'>Hi Matt
Marks being a little facetious
(because that course is unavailable in the UK at the moment) but that course would indeed cover not only nitrox but trimix for recreational diving up to about 37 mtrs.
From your post I do not see the benefit of doing a course such as PADI Deep Air Diver. I'm a BSAC diver and as sports diver can dive to 35m and as Dive Leader can dive to 50m on air.
The depths probably aren't as much of an issue as the gas you are breathing. 30m NDL 20 mins, 35 NDL 15 mins, 40 NDL 10 mins. (Buhlmann 30/90 Tables)
If you were doing a wreck dive and using a reel to tie in from the shot by the time you've tied in its nearly time to ascend.
What I propose is that if you looked around at other courses such as perhaps an advanced nitrox then you may be able to do the dives you want to do under that C Card and gain a real benefit in your diving, thats all.
WL