I don't think a decreasing new visits ratio is a bad thing. One might be able to spin it as a good thing.
Are you viewing this traffic as a whole? Because if so, it may not be an accurate representation. From the sounds of it, this is a big brand that you're working for. As such, a lot of your traffic is likely to come through branded terms. A decreasing new visits ratio for branded terms is not a bad thing - if anything, an increase in traffic and returning visitors would show that the brand is being trusted. In the real estate industry, this could mean that people are liking the listings on your site and are revisiting on a regular basis.
First thing I would do would be to set up some custom segments in Analytics (if this is what you're using, apologies if not). They're quite simple to do. I would set up 3 segments for organic search: brand, non-brand and not provided.
In your dashboard, click advanced segments and then the new custom segment button. Starting with brand, start with a "Include: Keyword" variable and type in your brand name. I'd add a couple of "OR" variables too, such as mispellings of the brand or nicknames/short names it has. Then insert an "AND" statement and select "medium", typing in organic in the field. You now have a segment of organic traffic purely for brand keywords.
For non-brand, do exactly the same, except for instead of "including" the brand keywords, you "exclude" them. I'd also exclude [not provided], add that as an "OR" variable.
For not provided itself, it's very simple - you just need one "Include:keyword" variable, at which point you type in [not provided].
With these 3 segments, you'll be able to see the new visitor rate of each one. I think this would give a more accurate representation of things. As mentioned earlier, I don't think a decreasing new visitor rate for brand traffic is a bad thing at all. For your non-brand keywords, a decreasing rate may suggest a decrease in your SEO visbility for some keywords. Sounds like you might be suspecting this to be the case. However, again, if traffic was still increasing I wouldn't be too worried. But a decreasing new visitor rate for non-brand search is perhaps more of a worry than brand search, so it's important to segment the traffic (in my opinion).
I think title tags do have a decent part in contributing click through rate, as does the meta description. I think it would be wise to clean these up and optimise them if you think they could be improved. Having said that, if impressions are down by 22%, it's probably your visibility that's affected, not the CTR.
Have you been monitoring your keyword rankings. Watching rankings every day can make you go a bit crazy, but it's a great indication of what your SEO visibility would be, which in turn is one of the best ways of getting new visits to your site. You're always going to rank #1 for your brand, so capturing new visits for people searching for london estate agents and similar search terms is what you can influence most. Optimising titles and meta descriptions is a good start, as well as increasing the rankings for some of the terms that offer the most impressions and are related to the business. Off-page factors are equally a big part in increasing this visibility. Not sure what your marketing/link-building strategy is at the moment, but I'd look to do more of that to increase your rankings in tandem with the on-page factors.
Hope this helps!