Cheers for the advice guys!
The monitor isn't a problem as I can see everything on it and the hdmi - hdmi connection has been tried and tested.
As for the usb - micro usb cable, would a faulty one still allow it to boot but not allow for any peripherals to be of use? If so I'll get another to see!
cheers again all
The RPi will probably boot OK with a low input voltage from the PSU because the CPU/GPU and memory are running from lower voltage(s), derived on board, which will hold up even though the 5 volt input may be low. The USB chip and your USB devices are connected more or less directly to the PSU input.
Do you have or can you beg/borrow/steal a voltmeter?
Another USB power cable of identical type
is unlikely to help. If it is the problem it is a design shortcoming, not an isolated one-off.