Fix that in with silicon or cement and it will look terrible. That would a typical "tradesman" bodge. Cut a square of treated wood and affix that with silicon and screws. The silicon should not be seen and will/should be used to seal the edge of the wood to prevent water ingress. Cut your hole in the wood and that's it. A bit more work but it will look neat. It will be weather proof and it wont look like a bodge job. Yes I do know tigercubrider suggested this as well.
Get a bit of stainless/ aluminium or wood/upvc and make a plate 50 mm bigger than the vent and fix it to the wall, with a 150mm hole in the centre
Must admit, my preference for heat transfer would be PVC because if you use stainless it will transmit the cold freezing external temperatures into the house faster, timber, if used should be pressure impregnated to avoid wood rot. Problem is using PVC the screw fixing into it can be a pain?
Other considerations?
1/. Mastic seal between the flange and the brick, watch out not to smear the mastic on to the exposed brick, a pain to get back off again.
2/. Pack the space between and all round the external of the vent duct within the wall thickness with Insulation that way you avoid a cold spot.
As for the gloves simple answer is that if the kit has to be returned no finger prints to incriminate the returnee ? just a thought ? after all you never know who is holding the finger prints paranoid or what? [were the vent kits made in any of the old Soviet Union states?
From Homebase / B&Q / Wickes - Could I get a UPVC (or PVC?) Panel that is about 180 by 180? If not - where?
It seems to me - the prioirty should be on getting a good seal to prevent water ingress over the long term? The panel has to be thick enough to transfer the load, to the mastic so it pushes up against the wall over many summer's / winters. Don't like the idea of using wood - as even when treated it will degrade and any resulting mess will only be visible after quite a bit of damage is done?