Hi King Tench, been rereading this thread and there are so many conflicting views that your head must be spinning. Trouble is, there are so many shelters on offer these days and each person has their own preference and budget.....
I had similar problems last season looking for an appropriate shelter to suit my own needs...
From what I can gather, you are looking for a day shelter, than can provide adequate cover for the odd overnighter....
Well it's a no brainer to me..... A Brolly System with infill panel is perfect....
On days, you only need the brolly..no groundsheet or infill panel, so you are not carrying needless weight around. If you plan an overnighter, then take the infill panel and zip it on.....
This will narrow your search down to just brolly systems, which you can then narrow down further to budget.......
I'd be looking for a brolly system, with a rear vent for those really hot days and muggy nights.......
Also, a mozzy mesh option on any openings on the infill panel makes life easier in mozzy season.
Up to you how much you spend, but unless you are a bivvy rat and doing 4 or 5 nights a week, you don't need to go overboard, beware the marketing hype of some products. Plenty of decent shelters under £200.
Some of the more expensive ones are sold on how light they are and as such are made from very thin material, which can leave you baking with no real shade in the warmer months.