We used Olive and Barr last year for a new kitchen - I've just seen this post as I did a search out of curiosity to see if others have had the same awful experience we did (!) Incidentally when we went for it, they did have reviews online, all 5 star but not very many. They have since disabled their reviews on Google - I imagine after the totally shambolic summer they had last year. It might be too late for the original poster, but wanted to add my two cents.
In short, the kitchen is actually good quality in terms of the build (interiors, drawer movement etc), although this is only the case after quite a few complaints on my part about cupboards that didn't close, trim that wasn't attached, edges that weren't sanded etc etc. They have also made some pretty fundamental errors such as installing the cupboards so close to the range oven that the doors of the range can't fully open.
Where it really started to go wrong for us with them was with the painting. At the outset I asked them about spray painting because I really wanted a perfect finish. Their response was that it was better not to so that you could change the colour down the line, and they assured me that the finish that would be achieved by their painters was so good that I would never know the difference. Sadly that turned out to be wrong - it took about 4 or more months and several visits of 2 days each and it still looks crap to be honest. Having originally had the kitchen installed last June/July, by November we still weren't happy so let them have access for an entire 4 day period while we went away for a wedding and our anniversary - we got back, hoping to find everything now finished perfectly, to find that they had done less than half the remediation, left a broken glass and spill all over the worksurface, and our brand new Dyson hoover had gone missing (though they weren't the only people on site so who knows about that).
Eventually and after my husband really lost his shit they sent someone else back, however even now because they had to come and rectify mistakes after the worktop had been installed, there are marks on the worktops, and paint marks on the underside of cupboards where it should be wood coloured. We also had to pay our builders (and buy more paint) to touch up the wall paint around where they had made a mess of it by using tape that peeled the paint off. These things are pretty frustrating and annoy me every day during the day, but when the lights are dim and we are eating with friends in there it looks good overall.
I am still awaiting a final piece of the kitchen that was supposed to be included in the price, and they've just stopped replying to my emails now :-/
In summary, honestly I would look elsewhere! They were cheaper than the competition but they honestly were the worst company in terms of customer service that I've ever dealt with, maybe it was due to teething problems in their first year...