I can now answer my own question after some research.
Yes, I can use one with a different voltage as long as the voltage is higher than the original.
I have a development....................
My vu has been unplugged all night as the Humax was still plugged in.
This morning I powered up the vu and found that I could not receive any HD channels.
After about 5 minutes of flicking through loads of channels to confirm that it was only HD channels affected, I began to get the HD channels again.
This backs up the info about C848.
If this capacitor is failing, when cold it can affect the ability to get any HD channels. When warm it can be working. Tests have been done using a hairdryer to warm it up to prove this.
Of course, when the box is on for a while, it warms up.
So, I am now off to my local Maplin store to get a capacitor. I hope that replacing this also cures my low signal on all channels, HD & SD channels.
Sadly, I will have to strip the box right down to get the board out as the capacitor is very close to the board surface and there is no way I could snip the legs of the capacitor.