Raised bed question.

If you line them you could run the risk of them filling with water when it rains and then the roots can't breathe, so I'd have the drainage holes so that the water can drain out the bottom. But....you will need to maintain a good watering schedule as they can dry out very quickly in hot weather.

Never tried this, but theoretically, if you put polythene on the bottom in one piece and let it go a couple of inches up the sides, this should act as a sump retaining water in a warm summer. If we have another wet one though, you could just stick a fork through the plastic to allow it to drain away!

I'd go with Snadger on this one. What about piercing the plastic maybe an inch or so up the sides so that in damp weather the excess can drain away. You would have to watch it carefully though in case it dries out too quickly. Perhaps line the inside of the base with damp newspaper or thick card before putting in the compost, in an effort to retain moisture?
How deep are the beds Patricia. You can now buy raised bedliners but, of course, that depends on what size/shape you have made yours. The depth will make a difference to retaining moisture as well as what you can grow.

One of the big advantages of raised beds is that their drainage profiles are better than level soil - all other things being equal, they retain moisture better during hot spells, but when the weather is much wetter (for example during and after floods) they drain more continuously and thus stay less waterlogged. (Worth noting that I am talking of fairly high ones here, not just a single board in height.)
I suppose it depends on soil type, but I would never use even a shallow impermeable plastic liner in a raised bed, in my experience it always ends up creating sour soil at the bottom of the bed which has terrible, terrible effects on plants ! Maybe if you kept a close enough eye on it this wouldn't happen, but I wouldn't bet on it. The first you know is usually when the plants start dying and you find the subsoil stinks. Once anaerobic bacteria take over, that's your soil b***ered for ages.
So what I would do for any raised bed on an impermeable surface, is create a permeable surface at the bottom. I would suggest a thick layer of newspapers, one full newspaper thick (nothing glossy, but other inks are soy based), with the wood actually sitting on top of them, and the edges slightly protruding from the sides and ends. This acts as a wick, either soaking up water from outside (you could water from below this way in a drought), or draining it from inside should the amount of moisture there be greater than the absorptive abilities of the cellulose. And as a side effect, the earthworms will love it, which can't be bad. Disadvantage: only works for one or two growing seasons, then the paper rots down.
That is perhaps not so tidy, but as a neater alternative you can just drill small holes in the side - you don't need many - to let out excess water in real downpours. That has the disadvantage of letting in slugs though, which might not be such a problem if they have an expanse of dry hot concrete to pass over, but is a real pain if they do get in during a wet summer.
Or failing that, you could do the lazy option - just drill some fingerwidth holes through the cement....?
Either way, I always put newspapers in the bottom, it's my biofriendly equivalent of swellgel.

I have bought Garland mini raised beds for my patio and they will be sitting on slabs too, they came with a liner which looks like it's made from the same stuff they make weed matting from. I guess it makes sense because it allows water through but retains the compost nicely.

Angela