Yesterday I read and considered the replies from friday night and decided to take a chance on the Xigmatec. I held on to one of the f12 fans, and installed it over the GPUs as an intake fan. I also persisted with a plastic wedge between the GPUs to aid air flow. Instantly there was marked improvement to previous. Without making any further effort, the top GPU temp only rose to the very high 80s, which was better than I originally would have settled for. By manually setting fan speeds to max, that temp drops to low to mid 70s. In short, it seems like problem solved!
I did appreciate (and now have a greater understanding of) the thought process that says a quality case is an essential investment, and I did consider whether I should have attempted to wait till xmas where I may have been able to afford the types that many on here were recommending. However, it was frustrating to have spent so much in recent months and be still unable to 'use' it to it's full potential. I don't have any intention to upgrade anything else for probably a good 2 years or so, and so I just wanted to 'get by' for now as quickly as possibly and felt there was a good chance the Midgard would allow me to do that and it has. So in short, while it may not be the best case, or meet all my future needs, it's been the right case for me right now, and I'm happy with my decision.
I just want to thank everybody who offered some advice or opinions in this thread. Even the ones I didn't necessarily act on, I appreciate the attempt to help out. Special thanks to Diggsy who really pushed to highlight the importance of the case, and continued to advise as my situation progressed, and to Surveyor for the simple yet effective suggestion of the wedge which I believe has also improved my airflow.
I'm a 'casual' gamer in the sense that with work and married life I only get a few hours per week to play games, so I've never had the time to really get into the 'overclocking' side of things as I just want to use my time to play rather than tweek every last fps out of my setup, so when I hit this problem I was coming from a very inexperienced and uneducated position to try and solve it. That's why I really appreciate everyone taking the time to help out, and I hope you understand why I went with the more basic option, and it was definitely not a case (no pun intended) of me disregarding the common consensus that a top level case would have been a more worthwhile purchase.
Anyhoo, while the main issue is now resolved, I would like to ask two more questions about my setup, and as they are related to cooling I think I can get away with it!
1. The Midgard allows for the bottom mounted psu to be installed fan up or fan down. I went with fan down without really understanding the difference. As It took me a good while to route the cables suitably around the case, and tidy them as tightly as possible, I won't be swapping it around any time soon, but if the common consensus says 'fan up' would be much better than I'll look to do it over the xmas holidays.
2. I've been using MSI afterburner to monitor temps and manually change fan speeds during testing. What is the common consensus towards managing fan speeds while gaming? Does everybody let them on 'auto' unless they have cooling issues, or is it recommended to use the like of MSI A to pump up the fans during gaming sessions to keep things running cooler? My current plan would be to switch on fans to 85% when gaming and switch it back to auto when finished. Difference in temps would probably be around 89 degrees on auto to 82 degrees on 85%. Thoughts?