I'm just really struggling to come to grips with "A £700 Delta Titanium will hold its own with a £1800 Ziess/Swarovski"
A lot of it is to do with marketing and branding and unique selling points. Zeiss always win glass tests, for example, whereas Swaro, it is said, will not allow the glass in any of their scopes to be tested against other scopes but they market on brand image and customer service. I've never seen a Swaro scope directly tested against any other and I've never seen on appear in any of the big, well conducted, comparative tests but, of course, their glass is adequate for our purposes which is all that matters.
The truth is that all of the "top name" scopes perform well beyond what the stalker needs, they are all more than adequate for your purposes and so by spending more you are mostly buying bragging rights and magazine adverts in your fave shooting magazine. You see this a lot with people posting on here effectively asking others to justify their spending a fortune on something they don't need by a big name manufacturer who has big adverts so the end user can tout the name around and try to impress people. This has left a huge opportunity for "second tier" manufacturers who can also produce adequate glass but who appreciate that by not going right to the limits of current technology you can knock the cost of a small car off the price and still provide the end user with an excellent scope. Due to the savings you probably also get rather less good customer service and less expensive magazine adverts but in most cases the optical quality is beyond adequate.
As you can often see the "big name" manufacturers have simply nowhere left to go and so they are selling on "features" which are things that look great in the adverts but, basically, either make up for other design failings or are simply rarely used or have such specialized application that only a small percentage of customers will have an actual use for them. Once they'd produced a good quality 6X42 or 8X56 that allowed you to shoot up until the time when you decided that finding the shot deer was getting difficult scope development was over and the development of marketing features and unique selling points started.
I haven't looked through a Delta but, clearly, neither have you and with that in mind you'd be very unwise to dismiss it as not comparing to the big names. Equally the best value in scopes is always a second hand fixed mag from a big name, because big names hold their money, as it has every feature you need without you having to carry and pay for things you don't need and will always be worth what you paid for it. On too many occasions you see people on this forum wanting others to justify their spending over £2k on a scope and then see them complaining about how they have no stalking and about the cost of stalking. If they'd bought a scope appropriate for their requirements, at £300 - £400, they'd have £2k to spend on stalking and the perfect scope for their job.
So, you need to go take a look at a few scopes and compare them at last light and then work out your priorities. By all means if you want a big name with fancy features then go for it, if you want something cost effective that will do all your stalking work for you at zero cost then go second hand fixed mag and big name, if you want something new that will do your job with a cash saving over the big names then don't knock the second tier manufacturers until you've looked through them as you will be doing something very different to any other stalker if they don't work for you.