Hi Alistair,
By 'stage monitor', I guess you mean something for the perfomer, not for the audience? (What is a VDU? Video Display Unit?)
I recommend a real floor monitor, with a shape specifically designed for that. Of course, you can put guitar and vocals through it. Otherwise, imagine how many monitors there would be on stage for a 10 pieces band. To do that, you will just need a 'monitor out' on the mixer.
That said, it's sometimes more comfortable to use a guitar amp as a stage "guitar monitor".
I don't know whether you need an amplified one, or just the speaker, but most makes exist with/without amplifier.
For a pub, I think you would be OK with 100 W, 150 would give you more headroom.
There are some very cheap ones nowadays (e.g., Boost), but an experienced guy (he's doing sound twice a week professionally) told me to stay away from them: they are not able to cope with voice dynamics, and they break very easily as a result (3 to 6 months expected life on average).
As a minimum, I would recommend mid-range. In that category, I like the Peavey Eurosys. Around 200 € without amp, 400 amplified.
Otherwise, the accepted pro standard nowadays seems to be RCF. Around 600 €, 200 W amplified.
Didier