The rest of the circuit looks OK to me. As the load is resistive, you could use a zero crossing opto triac such as MOC3042 instead of a random fire one, which would create less mains borne interference. But unless the heating element takes a lot of power, it's probably not necessary.
total power is only 1 kW (4.5 amps or so at 220 volts)
seems I'm actually using a MOC3080X - which is zero crossing
Yes, D3 is clearly redundant, since thats an optical diac in the coupler.
Is there any reason you opted to build your own versus use a Solid State Relay?
You can get a ready to use 10 AMP SSR from MPJA http://www.mpja.com/
, for example, for as little as $7.95.
parts cost < £1, SSR > £10
easy choice really - I'm a cheapskate by trade
no major reason other than it's part of a larger project
I'll have a look at what's available on this side of the puddle