Stayed for a (self-funded) business trip Tuesday-Thursday and was pleasantly surprised with how comfortable and well-equipped the apartment is. It is very much self-service - ahead of travel, you are required to send ID / pay a deposit, but having done so the check-in process was extremely smooth.
The Old Street StayInn is a short (15 minute) walk from the City, and in a very quiet area - I enjoyed two very good and uninterrupted nights' sleep - and the apartment was suitably spacious, well-equipped and clean to allow me to relax there on an evening, so as to enjoy a leisurely commute into the office the next day! Pret/Waitrose can be found a very short (2 min) walk away.
Admin team were responsive on email, and for the relatively low-cost for what is a prime location, I would highly recommend staying here, so much so I have booked again next week for the Aldgate property.