I cannot be of much use.
Hopefully someone else will jump in with info on this unit.

Without a model number finding any info will be impossible.
Even with a number it is doubtful.

This is a US site so info on UK appliances is scarce.

Here is a good site with help on fridge repairs though your p0wer will be different.
http://www.applianceaid.com/repair-help.html

Check the back of the unit there is usually a cardboard cover that will let you get at the compressor etc., remove the cover.
Unplug the unit and the connection to the compressor to see if your lights will now come on full.
If they do then it could be a compressor or it's start device problem.