No one can guarantee that a DSC1 will let you be granted a 243 on your ticket, but having it will certainly not do your application any harm.
Getting into deer stalking is going to lead you along a path where plenty of choices have to be made - calibre, rifle, ammunition, optics, knives, deer (species, sex, age, etc) and so on.
At the moment your choice is "how to acquire a deer legal rifle?"
IMHO you basically have two options.
Option A - as has been point out already, DSC1 is not a formal pre-requisite for being granted permission to hold a deer legal rifle. There will be plenty of people on the Internet who will point this out to you, and they are 100% correct. Some might even suggest that you take up the battle with your local FEO so that you can insist on your rights. Just keep in mind that few, if any, of them will be lawyers specialising in Firearms Law. None, I suspect, will offer you any practical help. All, I would guess, will already have deer legal rifles on their tickets. By all means follow their advice, and best wishes if you choose to do so, but citing them in evidence as the "good reason" why you should be granted permission to hold a deer legal rifle on your FAC is unlikely to yield much success. At the end of the day you may get granted your .243, but you won't have learned anything about deer and deer stalking along the way.
Option B - you say in your opening sentence that "I really want to get into deer stalking ". If that's really your goal then I'd suggest you focus on that. There are plenty of rifles for sale because someone was 100% convinced they wanted to get into deer stalking but then lost the enthusiasm or moved onto something else. In your case you say that you have already been on a paid stalk and accompanied someone else for the experience, which is a great start and shows real commitment, but as well as commitment what many FEO's will want to see is some knowledge and experience from the applicant when it comes to both firearms and the quarry you intend to pursue.
To get this knowledge you have another choice. You can carry on going out (either paid or with a mate) until you have built up a bit of a portfolio of experience, and then use that (plus evidence, perhaps, of some future paid stalks) when you submit your request. That's how I got my FAC. It's not cheap, though, unless you can find someone who will do it for mates rates or for nothing. It sounds from your post like you've already been down that path.
Alternatively you can go on a course like the DSC1, where you will learn about deer, the relevant legislation, firearms and ballistics, safety, etc. Yes, it costs some money that you could otherwise spend on stalking, but in the grand scheme of things it will be peanuts if you really get bitten by the bug! In addition you are likely to meet some like-minded people amongst the tutors and attendees that may help you on your journey. You will come out of it knowing far more about deer and deer stalking. There are lots of DSC1 providers out there, so have a look at their websites to see what the course covers. You'll also find lots of very useful information on this site. Keep in mind that all the approved providers follow the same curriculum, which is set (and assessed) by DMQ - you can find their summary of the course here:
DSC1 – Deer Management Qualification
Personally I enjoyed the DSC1 - I learned a lot, met some like-minded people, had a lot of fun, and made some acquaintances and friendships that have lasted me throughout my time in stalking.
I am sure others here will have different opinions, and different advice to give. My opinion is just my own (and you know what they say about opinions!), but whatever path you choose to follow I wish you success, both in getting your .243 and getting into deer stalking.
