The quilt looks super - well done!
Once you have the quilting done the whole top will stabilise, so don't panic about the seams. Maybe turn your "bag" wrong side out again and do a spot of hand stitching on the seams that are causing problems just to get them solid before you quilt.
If you're machine quilting remember there's no reason not to have a colour that matches the top as your needle thread and something that matches the back in your bobbin. One thing you should do before starting is make yourself a sample quilt sandwich that has the same backing fabric and wadding and a some of the top fabrics - play around with the machine tension to make sure that you don't get the bobbin thread showing on the top and vice versa. Don't use the auto tension! I use 2-2.5mm stitch length for quilting. No need for a special needle, but I'd use a fresh one.
Start your quilting in the middle of the quilt - this will give you chance to even out any excess fabric between the middle and edges. When I start a line of quilting I like to hand wind the needle down and about half way back up and pull the bobbin thread to the top - this stops it getting caught and making a mess of your back. It also means you can "bury" the needle and bobbin thread together! Thread burying is the one chore I hate, but one thing that makes it easier is to thread a hand needle so you've a loop of thread. You insert the needle at the base of your thread tails, run it through the wadding about an inch and then catch the thread tails in the loop- pull them through, trim them and hey presto. MUCH quicker than threading each tail into a needle.
Sorry, that's all a ramble - good luck!

Oh its not going so well ; I have been having problems as some of the red fabric is really cheap and keeps fraying - a couple of my seams came apart a tiny bit in places (boo hiss). What is the best 'patch up' method if it happens again?

I have 'bagged' the quilt now though, I went with dizzyday's simpler method. Am apprehensive (to say the least) about the quilting part. How do I sew it accurately along the seam - I'm not that accurate. What colour thread should I use; do I match it to the front or back?

Any advice on stitch length, needles etc..?

It's lovely, well done, I've been making "stuff" for years but never attempted a quilt yet!

You should be proud of your efforts!!!

Oh you are all too kind - it obviously looks better in the photo than RL!

I am still trying to find time to concentrate on quilting it etc as I have an interview to prepare for tomorrow (not sure which is more scary tbh)

WOW - For a first quilt it's marvelous

Its lovely I would bind it too, and quilt in the ditch.

I've never had a machine that behaved for machine quilting, and as I find quilting by hand soothing I do it that way. But then I'm an old school english pieced over paper kind of gal - mostly as I sew when I'm travelling for work and it's portable

Don't have any technical advice as whilst I like the piecing bit I have always got my mum to do the finishing for me (with nice binding round the edges), but just wanted to say your quilt looks beautiful! If it looks this good now it will look even better once it's quilted.

That looks great. I'd bind it with the red or blue. Do you know about this group? www.flickr.com/groups/quilts/

I have added a photo of the quilt so far. Please feel free to offer advice on how I could improve it . It is far from perfect, but never mind.

The pfaff has an extra bit <technical> on the back that supposedly feeds the fabric through without shifting the layers.

I love my machine, it is old but so easy to use compared to my crappy old Toyota one.