I just multimapped your location, two of the results I could point you to near(ish) places, the others will need a trip to pick anything up!
One I do know of (tho' probably a bit far north for you) is Ardoran near Oban has (had?) a Honda 50 4- stroke for around £1500.
More southerly, there's Walkers Chandlery in Leigh - on -Sea (Southend, Just off the A127) has an assortment of secondhand stuff on a rack in the shop from 2 - 200 HP, what hangs on it varies from weeek to week. There's another in the Southend area the name of which I forget.
The other way of looking at it - What's your budget? £400 for an engine + £200 to get it properly serviced = £600. If you're looking in the " few hunderd" cateory then realistically there isn't going to be a lot out there that is going to be "plug'n'play at that price. Also if it doesn't idle you can haggle him down a bit, freeing up more cash to get a service..... Engines of that age don't depereciate unless you blow them up properly & totally kill them, so worst case, you can use it as a trade in for a known runner if it turns out to be knackered? .....And if its got PTT I'll give you some cash for that bit!
I say all that based on my "half the fun of an old engine is bringing it back to life" theory!