Please can someone enlighten me as to what the difference is between this and a standard plate? I've looked on Google Images but can't find a clear enough picture. I've been reading through some of the recent advice on how to persuade a Janome to do free motion quilting and these plates were mentioned several times. Are they available for all models or just the expensive ones?

They are available for most machines.
The single hole plate has just a small round hole in it, it is for straight stitches only. A more standard plate has a wider slot to allow for zig zag and decorative stitching. :D:D:D

A single hole plate is just that, a small circular hole just a bit bigger than the needle to go thru. The other plate is a zig zag plate where the opening is oblong to allow the needle to go down in various places as it makes the zig zag (or decorative) stitch. With the larger hole plate, your fabric can actually be forced into the hole, people say that it's 'eating' the fabric. This may jam your sewing. The single hole provides more support for the fabric giving you a better stitch quality. If your machine only does straight stitches, you probably have a single hole plate. One caution. If you do change to a single hole, put a sticky note or other identifier on your machine so you don't change to a non-straight stitch. Best case, the needle will break or bend, worst case, you will throw your machine out of time requiring an expensive repair. You would have to contact your manufacturer to see if a single hole plate is available.

Yes what Amma said and in general used for stitching smaller peices like we do when we peice a top because the smaller peices sometimes get pushed down into the longer hole and cause all kinds of cussing. So I guess you can say the single hole plate is a cuss buster plate.

Also - if one forgets that one has a single hole needle plate in/on the machine - and tries to do zig-zag stitching - the needle can break and pieces can fly up to your face and eyes.

If a needle does break - try to find all the pieces so the whole length of the needle is accounted for. Don't want pieces on the floor or in the guts of the machines.

I'd just as soon not explain how I know this. :oops:

You can often find an older straight-stitch only machine for less than the cost of a striaght stitch needle plate. I do my best piecing on thrift store machines.

here's a nice clear pic. See the hole where the needle goes down into the bobbin area? It's only a small round hole as opposed to a long slot.

That's a brilliant picture- thankyou. I'll have to find out if something similar is avaailable for my Janome.

There is one for my machine, though it's not cheap. Is this something that might help with tension problems when doing fre motion quilting?

Absolutely love mine! And my machine has a setting that makes a warning come up when you first turn it on to tell you that you can only do straight stitch at the moment.