We used them 5 years ago and they did an okay job in the end but I wouldn't recommend them.
Simply Extend gave us a what seemed a good value quote but there will be hidden extras and also sloppy work, low skilled workers and workers not showing up and going over the agreed timescale.l for completion.
If we could do it again, we would go with Doran Brothers. Everyones extension I've seen who have used them look very high standard. The company directly employs their workers and they seem to work with integrity and to high standards and owners live locally. And no hidden extras!
Basically, really examine the quotes you are given. The cheaper ones are not always actually as good value as they may appear.