The standard figure-8 section of my power cable is broken, and it's marked as 7amp/125volt. I haven't been able to find any others marked for 7amp, so would it be safe to connect a similarly shaped cable marked for 10amp?
This is the type of head: http://www.kenable.co.uk/images/fig_8_plug.jpg
As 10 amps is higher that 7 amps then yes, you will be fine.
The 10A is the "safe" current carrying capacity of the wires, i.e. they can carry 10A without overheating or causing a potential problem. If one set of wires can carry more current than another set of wires then they are "better" and should, technically, be slightly safer.
If it were the other way and you were replacing a 7A plug with a 3 or 5A plug, then I would be slightly cautious.
