Usually you can make a supporting frame from 3x2, and t&g softwood flooring for the platform. Make sure you spread the frame at least one ceiling joist beyond the large tank to spread the load across the ceiling. The only caveat is you need to ensure the ceiling joists can take the load, it’s normally not a problem. I’d screw everything together rather than nail it and stay slim!

   

Ive had a think, and this is what I've come up with.  Hopefully this makes sense...

4x2 secured to the chimney breast.  I'm guessing something like 4" number 10 screws for added overkill.  Then on top of that I've got lengths of 4x2 at 14" centres with a ¾ ply top (I've got a sheet spare, could double this if needed to make 1½ thick)
The front of these is supported by a 4x2 running parallel with the chimney wall.  At either end, a 4x4 leg made from 2 lengths of 4x2 glued together, fixed with a bridle joint, glued and screwed from both sides.

The legs rest on a 4x2 which sits on 3 joists.  There is a supporting wall more or less dead centre under the tanks, and its not going to be very different in weight to what is already there.  

I think I'd also probably add a couple of triangle blocks to the bottom of the legs to hold them vertical.

Does that sound the sort of thing?  Anything I've missed?

Cheers

I don’t think you need 4x4 legs, 4x2 is ample. Personally I wouldn’t screw it to the chimney,  I’d build a complete frame, 4 legs with diagonal bracing, and I wouldn’t use ply even if I could get it through my small hatch.

One last question - I was planning to replace the tanks at the same time.  The lid on the larger tank has cracked and the smaller one has a huge corner cut off it for the overflow pipe, rather than a small neat hole with a seal.  

The tanks that are there are this style, sort of flimsy plastic
https://www.toolstation.com/water-storage-tank/p38349

But I can't find them in larger sizes.  Everyone seems to be stocking this sort  https://www.toolstation.com/25-gallon-polytank-blue-water-storage-tank/p91853

They do look much more rigid, are they now what you're required to fit?

Theyve only got the 25 gallon in stock locally or for delivery

thescruff wrote on Jan 3^rd, 2021, 9:03pm:


That takes all the sport out of it 😆
Seriously though, short of moving the hatch to a different room, it is as big as it can be due to the layout.  Walls in the way on 2 sides and timber frame for the roof and the landing light in the way on the other 2.

woodsmith wrote on Jan 3^rd, 2021, 9:12am:


+1

Take the load down to the joists rather than rely on screws into the chimney.

All done and not leaking  

I'll take a photo to show you what i've done once i've finished tidying up.  I did forget about the heating header needing to be raised up a bit, but that can wait until another day.  i'll need to drain the system again to swap some radiators so i'll raise the header at the same time.  I need to crack on with a few other little jobs before they call me back to work.  

Thanks again for your help, it is very much appreciated  

Yes, I know its hugely over-engineered, but I went with using wood that I had kicking about in the workshop where I could.  But it can quite happily take 18 stone of me jumping up and down on it, so I'm confident its going to support the tanks ok  

I still need to tweak the vent pipe slightly - I ran out of new 22mm and the re-used stuff is a bit crusty inside.  It could also do with being a touch higher as it does dribble ever so slightly when the pump changes speed, but the tank doesn't get warm like it used to from the constant flow out the vent.

Just got to sweep up and cart all my tools back down now

Thanks again for you help