In case anyone's doing similar, and as a reminder if/when I do this again, the cover removal sequence that I used was as follows, requiring a ~6.5mm hex driver for the majority, and an 8mm hex driver for the remaining two heavier fixings. E&OE and YMMV.....
1) I removed water collection bottle from the base. 2) This allowed access to two self-tapping screws that fixed the lower part of the front cover. 3) Removing these meant that the front cover could be lifted vertically upwards off two slotted holes (EDIT: after removing the rotary control knob). 4) The top cover came next, requiring a flexible driver to access two self-tappers at the front which screwed into speed fastener style captive nuts (don't lose those!). 5) The back of the top cover sat on two metal dowels, so just slid off after removing two self-tapping screws from the back panel. 6) Removing a small internal panel allowed access to the heat exchanger coils from the front (EDIT: 5 screws). (This can be done after the next step.). 7) Laying the appliance on its front allowed the two side panels and base panel to be removed as one assembly. 8 ) Finally removing the rear panel provided access to the other side of the coils.
EDIT on re-assembly: re-attach the control knob and re-centre before tightening the two lower screws to prevent binding.
In terms of cleaning, these were my observations. A hand vacuum was useful for general work, but a small brush and a rag was needed to remove a congealed skin of dust/detritus from the coils, some of which was still damp. I also used - quite gently - an old toothbrush and a microfibre duster. The fan blades were covered in a significant amount of collected dust and fluff, but as the fan pulled off the spindle this was easy to access. The side grills and casework got a good scrub. Internal sealing strips were perished, as were other foam inserts.
Things to have on hand next time: a few feet of approx. 6mm window-type foam draft excluder, an air blower with a foot or so of flexible tubing, a selection of el-cheapo long thin 'feather' or microfibre detail dusters.