hello!
We used Wren last year for our kitchen, and I have to say, our new kitchen is totally gorgeous.
We visited a branch in the next county to us, so they didnt / wouldnt come out and measure up our kitchen, but we made sure we had very accurate measurements / details of all pipework / plugs etc when we sat down at the showroom to plan our new kitchen. We were there hours with the planner, making sure everything was right (kids dvd entertainment provided!).
They were very efficient at scheduling our delivery (called a week before due date to state exact day, then called 24hours before due date to state exact time, then had text message on delivery day confirming time). We checked every item off the lorry to make sure we had it all.
We didnt use their fitters - found our own on Check-a-trade, and we bought our own granite worktop from elsewhere as it was slightly cheaper.
We did need a few more bits and pieces once fitters had got going with the kitchen, so we telephoned and ordered them - took a few weeks, but they kept in good contact and our fitters came back a few weeks later to finish off.
Everything was absolutely brilliant, went very smoothly, and we're really pleased with our gorgeous new kitchen. Fitters commented on the good quality of the pre-assembled units.
HOWEVER.... we recommended Wren to a family member, they visited a different store, and appeared to have had a totally different experience - stuff missing / damaged on delivery, rubbish customer service etc, measurements weren't right so stuff didnt fit. However, this family member is slightly odd and fussy(!!). The damage to the units didnt appear massive in the pictures she sent to us (ok, so you don't want damage if you're paying £600 per unit, but it was slight scratches or dirt marks on the bit of unit that would be covered by a worktop anyway!). Plus I couldnt understand this family member complaining that the measurements werent right - its YOUR responsibility to measure and make sure things fit if Wren don't come out to the home to measure up. We had no problems with that side of it at all. Plus after complaining, Wren gave them £500 compensation, which seemed to cheer them up and do the trick!
Phew, long post! In summary, if you are totally on the ball with your kitchen measurements, own the whole design, check and double check using the plans they provide after you've ordered your units and complain quickly if anything goes wrong (the family member just sent emails to Wren rather than pick up the phone as they can't bear confrontation - we always telephoned Wren with any questions or when ordering extra stuff and got immediate assistance), then I'm sure you'll have no issues.
HTH!