Well, basically got an email back saying they are more than happy to help, however they couldn't tell me how compatible the rifles are (???) e.g. are they TM based. Although they did say that they do manufacture a 120rd midcap, but they do not have an agreed distributor as of yet to sell them, so they offered me to buy them direct from them. I have asked for more information, and explained that it may not be best price-wise to order from Hong Kong due to import tax, however if they can offer me a good deal then I'm willing to think about it, as someone said above, can't go wrong with same-brand mags surely?
Most mags I see are first off TM compatible, however obviously I know different brands of guns have different specs. for their magwells. I did find a cheap mag on taiwangun, where in the description they state 'Medium capacity magazines for M4 series fits to AEG replicas in Tokyo Marui standard'. My rifle is listed below this, so I assume that I can take a chance on any mag that is listed as being TM compatible. The mag itself is steel constructed 150 rounds for £7.27 per mag, obviously Chinese made.
Dunno if im willing to take the risk on spending 50 odd quid on those mags for them to only last 5 mins as they are pretty cheap.
I am more leaning to King Arms mags, I have seen on fire-support they have a box of ten of their plastic 120rd mags for £49! Haven't found a deal like that anywhere else.