I go with whopity
But not only does it go towards your 900 but also 100 in 28 days. And all the other days off counts as well.
Although I must admit I have seen an CAA AOC holder's FTL scheme where it only states the sub 5700kg rule. ie if its under 5700kg you don't count it.
This could be where the confusion comes from.
To be honest as nice as it is to think about doing some instruction. After flying 100 every 28 and 900 hours a year I doudt very much you will be so keen to strap a C150 to your backside on your days off.