I had constipation for the better part of a week after my first chemotherapy session. The pharmacist supplied me with a box of Laxido
http://www.lloydspharmacy.com/en/laxido-orange-powder-30-sachets, telling me that the normal dose was two sachets per day but that could be adjusted to one or three per day, as needed.
However the powders didn't seem to be working and, following a bit of googling, I realised that I'd got faecal impaction. The leaflet that comes in the box of Laxido says that the dosage in such a case is 8 sachets over 6 hours. So, from midnight to 6am I took a dose roughly every 50 minutes.
Nothing seemed to happen for several hours, apart from me feeling rather ill and feverish for about half an hour around noon. That quickly cleared though and, at about 2pm, things started to move very quickly - literally! Over the next couple of hours I had several visits to the loo, at the end of which I was at least 6 kg lighter than I'd been before! Probably understandably, I felt a bit weak for a while but by mid-evening I felt really great.
So my suggestion is to keep a box of Laxido handy. It will come in useful in future both for normal constipation and for faecal impaction.