Right - tonight's update ... which should really have been at the weekend but I was tired! 
So - Saturday night I realised the upper sections of the subframe arms where they join to the bulkhead beam were a little suspect too, so as I said I did some digging. One was entirely shot through so I cut out the tin-worm and Sunday morning made up another patch piece:
subbie_upper-patch by Sabre75, on Flickr
The other side needed some work, but it was only the overlapping piece that was knackered, so I cut it back and ground away the junk to allow me to butt-weld the 2 pieces back together. And yes that gap is a little wide in the middle - I was a bit gung-ho with the cutting disc there 
subbie_upper-overlap-trimmed by Sabre75, on Flickr
So here's the patch plate plug- and butt-welded into place. As you can see the 'flat' upper section has been patched but I left the angled section as it was ok; I did seam-weld all around it though (including inside the arm) to prevent any water getting into the overlap and causing the same kind of problems again.
subbie_upper-patch-tacked-and-seamed by Sabre75, on Flickr
... and here's the other arm all welded up too; you can also see the seam-weld penetration from inside the arm ... and my foot.
subbie_upper-overlap-fully-welded by Sabre75, on Flickr
With all the main welds done I then seam welded all the flanges & edges inside the subframe around the areas I'd done the repairs; these weren't all perfect as it's tricky to clean the areas properly before you weld, but I got it done. One I've cleaned the frame for painting I'll go over all the seams with seam-sealer too, just to be on the safe side. But anywhere, here's where I am now - the subbie is all welded and hopefully won't need any more attention any time soon!
Here's the top of the frame:
subbie_upper-completely-welded by Sabre75, on Flickr
And here's the underneath:
subbie_lower-completely-welded by Sabre75, on Flickr
So apart from entirely rebuilding the top and bottom sections of both arms, the subbie was fine! - sealer and painting next ...
And Shawbags; cheers for that mate - I decided a while back that knowing how to weld would be handy, so I did an introductory evening course which I'd highly recommend to get the basics down and to learn the proper techniques from a expert. I then bought a cheap-y welder from ebay, which I wouldn't recommend. To cut a long story short the wire feed wasn't smooth so I could never run a decent bead, but I didn't know enough to realise that! Recently when my old machine died completely I went out and bought myself a Sealey MightMIG 150, which is a decent piece of kit and worth the £200 I paid. Apart from getting yourself a decent welder (and an auto-darkening mask; it's invaluable) it's just practice after that ... apart from the mini here's my practice piece ...
trailer_frame-in-progress by Sabre75, on Flickr
... extending my 3.65m trailer out to 4.4m (and totally rebuilding what turned out to be a very moth-eaten frame) - hopefully it won't snap in half once it's done! 
Right - need to sleep again - cleaning, sealing & painting next - watch this space.
I've started writing a project thread on it now (!) so I'll post a link to that here later.
