Simon, the majority of the mounts seen last night would have been made by the photographers themselves to fit the images they are presenting. It is possible to buy pre-cut window mounts and I have seen them at Clas Ohlson, but they tend to be expensive and the image would need to have proportions to fit the mount, rather than the other way around which is possible with a bespoke mount. The problem being that once the image needs to be cropped due to aesthetic considerations, it is quite likely that the resulting photo can no longer be produced with dimensions which are compatible with a predetermined size.
http://www.clasohlson.com/uk/Canson-Passepartout/Pr313997003The most common size is 40 cm x 50 cm (which is very close to 16" x 20" in old money) and this is frequently a requirement for exhbitions and external competitions. Mount board can be purchased as A0 or A1 sheets and then cut down to size, although it might also be possible to buy ready made 40 x 50 boards. In essence, mounts can be produced in on one of two ways. The photo can either be stuck directly onto the board or a window with a bevel edge can be cut into the board, which often looks better. A judge should not be influenced by the mount when awarding a mark as they should be looking at the image, but good presentation can be a factor.
It is possible to cut window mounts entirely by had by using a scalpel, but the thought of using something so sharp worries me. I like having fingers and find them very useful.
Most people use some form of mount cutting eqipment which makes life much simpler (and safer). The one I use was made by Longridge and I get good results, but it is quite expensive and there are other options available at lower cost. Google will be your friend in that regard and no doubt there will be some "how to" videos somewhere on YouTube.
http://www.longridge.co.uk/Another factor is the board colour, which can enhance or degrade the overall impact of an image. A few years ago, the vogue was for black mounts, but nowadays mono prints tend to look best with a white mount and colour in a cream one. There are exceptions and it is possible for a coloured mount to complement the hues in the image. For example, a blue mount for an underwater picture might work, although green for the same photo would look terrible. If it is not possible to purchase a ready made mount of the desired colour, this would be another reason for making one's own.