I would reckon the immersion heater element thermostat you already have MIGHT be worth a look - after all, they usually go right through to 95C and if you are wanting to boil mash you could have a bypass switch for them - but otherwise have them set down in the severnties?
The Phase angle controller suggested by Capt-Cudellez sounds like a good plan too though a shade on the pricey side. I am guessing this is a TRIAC device again (dimmer design) but I am not sure.
As at 750W, even a 2.2KVA (intermittent rating) site transformer would give you some help here - and although bulky can at least be had for about £20 second hand if a bit tatty and it is local enough to collect.
( Thinking out loud : - It DOES occur to me, that if people combined skills (copper pipe working, lid modification to standard tea-urn type boilers, electrical control etc), between a little group of us, we could do "kits-o-bits" to help people build their own reflux stills. The electronics for safe heat control (thristor triggered triac and a bit of a basic filter etc) really is "change from a fiver" sort of stuff if made in any modest quantity. It is the time taken to make a one-off and the lack of one-off components that make it hard for others. Making a batch of units - either ready assembled in boxes OR as a PCB to fit inside a metal "back box" and front plate that they provide and it becomes less of a headache.
Then other folks could concentrate on making the copperwork while another person modifies the boiler lids to enable them to have a cap for normal tea urn use, a bung for mashing or a reflux top for stilling. Someone else sorting out the water cooling coiling / temp control and a final person (or the same one doing the heater control) making a low hysterisis (for accuracy of temp maintenance) trigger to work the heater controller. With everyuone doing just one or two bits they are good at, it might be interesting to find how inexpensively (but still making a little to make it worthwhile as well as fun) a version of a T500 could be made.
The unmodified boilers seem to be around £55 for a 26 litre one, add to that the cost of copper pipe and £10 of control electronics (for heater control and thermostat) and you are probably not far of "base cost". I would guess there is room for a fair profit for work and still give an amazing deal to someone wanting a ready-to-go mash boiler-come-still. )Finally as a reward for putting up with my waffling and "thinking out loud", I have just found this link while hunting for boiler prices - it might be of interest to you.
http://cgi.ebay.co.uk/GENUINE-BURCO-HOT ... 1e557a0f68It is a 30C - 110C adjustable thermostat and I suspect if you ask them, it might be rated for 3KW. If not, then it could be used to simply switch a 16A / 20A relay for you. If it ISN'T rated for 3KW, then message me for my address to bung me a large SAE and I will donate a 20A mains rate relay to the cause - the coil will be 12v but this means the thermostat need not be securely housed (low voltage only on it). You will need a little 12v power supply though (or anything from about 10v to 16v will do if you have an old plug-top power supply from an old gadget that long since died. I will supply the relay with a metre or two of soldered "mains tails" for live in and live out and then some thinner wire to connect to the power supply and thermostat. All it will cost you is an SAE (but make sure it is a LARGE SAE stamp or the buggers will sting you for under carriage.
The only "irk" is that I do not know what the hysterisis is on these things - no use having it cut off at 79C but then not come back on until it drops to 65C - but if it controls within a couple of degrees, it ought to do very nicely indeed.
Mark
ps:
Although I have only a couple of heavy current 12v relays to hand, if anyone needs a relay with a 24 coil rated for at LEAST 16A (some 30A), I have a few kicking about from a project I have done. If you need ONE, bung me an SAE with a LARGE envelope stamp on and it can be yours

If you REALLY need 30A, you can have one of those, otherwise, it will be whatever I find first out of 16A, 20A or 30A

It just depends on whether you have a little old power supply that can provide at least 20v to work the coil.