Do you know how hard your water is? kH in particular would be helpful, but if you don't have the test just go to your supplier's website and look for the "Water Quality" page. Type in your post code and you should get a brief report along with a link to a full report. Look for the hardness figure and let us know what it says.
To help set your domestic appliances, the water hardness in different units is: 265 mg/l :Calcium Carbonate 106 mg/l :Calcium 18.444 ?C :Degrees Clark 26.5 ?F :Degrees French 15.052 ?dH :Degrees German 2.65 mmol/l :Millimoles
This is what comes up on Anglian water page.
I have had 'white fluff' appear in the last few days on the gravel, and on some ornaments and on my air tubing. Also, it looks like some algae is forming on the glass. Should I leave the lights off?
With that water chemistry there is no reason for this not to have worked. I once read a review where the tester had exactly this situation happen to them. Although Waterlife never reveal what is in their products Biomature is just an ammonia salt and a few other bits to create ideal conditions for a culture to develop. But essentially it is just an ammonia salt.
This is just a thought but did you use Prime to dechlorinate the water when starting this cycle? If so I wonder if it reacted with the biomature to cause this.
I don't think it's to do with the Prime personally. Many, many people (including myself before I switched to using RO water) have cycled tanks with ammonia and Prime and I'm sure there are others who've used Biomature in combination with Prime.....