I'm very new to moulds etc but found a simple work around was to cut and attach a piece bag to the back of the mould in the same way you attached to the front, found this a little easier than bagging the whole thing and can just leave it attached after.

Well ... maybe on very big vac-bag operations there will be some microscopic holes in most bagging films, but I have infused and vac-bagged hand layups up to a few square meters with NO vacuum drop over 25 hours, using the standard bagging film from EC ...
I bought some of the stretch film from EC .... I've never managed to get that stuff "vacuum proof" --- It does not even work as release film, quite the contrary ...

I have had some trouble vac-bagging on epoxy moulds using the Epoxy putty from EC ... but after painting the back side of the mould with infusion epoxy, and bagging it up with nothing in the mould they are now all leak free.
I did not get visible resin through the pinholes into the mould itself in any of the ones I "sealed" this way.
Most of these moulds are so small that I just envelope bag anyway, but I made a few that I wanted to be able to use without enveloping them.

"Sapere Aude"... Dare to KNOW!

The written word is the only truly efficient vehicle for transmitting a complex concept from mind to mind...

103% of all people do not understand statistics...

Do not adjust our mind, theres a fault in reality :-)

Sorry, but I spent years building moulds for the wind turbine business.  Our drop tests were robust to say the least and only when we stopped using generic vac bag from all the major suppliers and used a material that we sourced from a different industry did we stop wasting time and money carrying out drop test after drop test.  The manufacturers guaranteed the film had no holes but we proved otherwise.  They may well have been due to handling errors, but on a 50m long bag it is very hard to prove either way.  All I know is that when we moved to drop testing with our own bag we achieved  figures of less than 0.1mb per minute.

Bah, that's such a myth. Vac bags don't have holes in them unless they were put there by someone or something. No vac bag comes from the factory with holes, microscopic or otherwise. Have a look at how they are made, you will know it is impossible. 

Virtually all moulds will leak to a greater or lesser extent.  resin won't fill the pinholes as air is being sucked in not resin being pushed out.  Pull a vac on mould and flanges and paint on hotcoat.  Vac should pull hotcoat into any pinholes from the back side of the mould.  If in doubt just envelope, safe every time...... unless your bag has holes in it, but thats another story as most vac bag has micro holes in it that can f**k up your infusion if you aren't vigilant.

If you didn't see a big hole in the mould then it will be a tiny pinhole, so even if the resin filled it up then it will just snap off very easily, I doubt you will have an issue. 

thanks all, I'm hoping the resin hasn't gone into the mold and locked. Other than that, I'll smooth the back and try again. Will update with results in a few days

Happens more regularly than you might think. You might find that after infusing with a leak in the mould, the resin will find its way into the void and seal it up. No sense relying on that though, you may as well coat the back of the mould with a flow coat, and it would be prudent to sand it down and make sure the back is smooth before you layup again, just in case you do need to envelope bag. 

I would follow the idea of Chris as well just to be sure. 

Sand the back of the mould to remove any high spots and sharp edges then envelope bag next time, I also cover the back of the mould with breather.