I'm moving the 50 G cold tank from the kitchen to the loft. I want to place
it on platform suppourted by two high (head height) purlins (approx
17"x10") against the end gable wall (house is end terrace).

I have bought some 47mm x 147 mm joists which can support a dead load of
127 kg and an imposed load of 153 kg across the span of 2.7 m, with joists
at 400 mm centres. Total 280

Weight of water in tank is (if all of the tank is full with 50G?) 246 kg

I'll put 3 joists at 300 mm centres.

Does this seem ok?

Will I overload the purlins?

Alternatively;
Should I attach a joist to the gable wall, a joist between the purlins and
then joists between these - so that half the weight is taken by the wall?

If the alternative is recommended, how do I attach the joist to the
brick wall - Screwfix fixings are on page s 26-28.

Thank you,

Neil