How much will a robot improve my current level of play?

They are not necessary but it is good when you dont have a club all a training partner

If you are going to buy it, you should buy a one that does not have a lot of function. More functions will make it more hard to use and pricely.

IDK your budget but newgy 540 or 1040 are pretty good

For one, robots are practice partners if your are alone if you can't have a returnboard. Robots have the tendency to plummet the ball towards you giving you a feeling to hit them but only at a certain time (depending on your robot actually). And also, robots are the practice partners incase you can't multiball feed yourself. 

The disadvantages of that is that robots are costy and robots aren't that much of a challenge once you actually get used to it. They just serve there as ball feeders but actual ball feeding isn't necessary that much if you're doing it in an actual person or a more challenging level. 

Which is why I'd like to recommend a returnboard instead. Returnboards have rubbers, which builds almost the exact same level as you do because you are fighting opponents with rubber. Returnboards may cost a little cheaper than robots too, and that you don't need multiple balls to train. These returnboards when practiced with, practices your topspin even at a longer rally range. You can adjust the distance of it to maximize more power, and along with topspin practice, robots usually give just flat contacts of ball.

Your choice.

Thanks for the input Cloud. I'd never even heard of a return board but I've looked it up and found this: http://www.teessport.com/table-tennis-coaching-c18/tsp-table-tennis-return-board-pro-p837. Doesn't the return board mean you have to be accurate to ~30cm or however big that board is? I think I'd end up a little frustrated - at my level - if that were the case.

No, it doesn't need to be that accurate. You can look up at some DIY Returnboards on Youtube. Don't worry, its a matter of preference.

Also the link that you showed me, isn't the actual returnboard I am referring. Returnboards are actually two long boards covering the width of the opponent's side. 

Yeah but a real coach is mostly the best

but backspin balls impossible to return,you can to do one it's easy

Return boards are good for practising counterhitting or topspins, but they can feed you backspin balls, which is what Duncan wants to learn returning.

Which robot? I wouldn't buy one that is too basic. The first prices are sometimes not even able to feed both topspin and backspin balls, they can't change direction, etc. Around €500 is a minimum I think. Buying second hand could be tempting, and I'm sure some people buy robots, use them a couple of times, get bored and/or lazy, store them for some years and finally sell them for bargain prices. But be careful, it's really hard to judge if a robot is worn out or not. We have one at our club, it looks very good, but it has multiple small plastic gears inside that are totally worn out, and it's close to useless. So if you buy one, test it, and if you really want to be sure (and the seller allows it), remove the covers and look at the gears, the drives and the rollers. Also make sure that it can be used with 40mm (and now 40mm+) balls.