Can I T the return into my breather, so I don't have to add another fitting into my tank?
I only have a breather that goes to the filler neck.

wouldn’t think so, if you blocked the return line (by fixing it to the sealed fuel tank) then it would be at the same pressure as the main fuel line

No the tank isn't sealed as such.
I need a return to it from a swirl pot, so was asking if i put a T piece into the breather which is on top of the tank and is just a pipe up to the filler neck would that cause any issues?
If it does I will have to add another fitting into the tank for the return feed.
The breather has no one way valve in it, just a pipe to the top of the filler neck.

Not ideal I wouldn't say,as surely if you have a constant flow threw return you will effectively be stopping your tank breathing?

When I went EFI I used the breather as the return and fitted an Aircraft fuel tank connector at the base of the filler tube, for the breather
http://www.parts4aircraft.com/fuel-tank-fitting-c2x9383165

The tank must be breathing through the filler cap as I have no other breather.
I don't know why I fitted breather pipe from the top/middle of the tank to the filler neck which is in the top rear corner of the tank anyway? the breather is essentially redundant isn't it! I could have just not put a fitting in the middle and blanked off the one on the neck couldn't I?
I suppose I can try it blanked off with my return to the old breather and if i have issues replace what is there now and add a new fitting.
Thinking about it more the breather now would only help remove any air at the top of the tank whilst filling up, but it's a flat topped tank so shouldn't be an issue.

If the tank breathes through the filler cap ( like mine does ) then it'll be breathing through the filler hose anyway. Just feed the fuel return directly into the breather connection on the tank and forget the breather. That's what I have done. I have restricted the low pressure flow to the swirl pot and therefore the amount returning.
The only downside has been that I have to refuel a little more slowly to allow the displaced air to come back through the filler.

Yep I'll give that a go. I don't mind taking a bit more time filling up.