Since MacLaren are not providing replacement wheels for older models (which is ridiculous), here is a solution for replacing wheels using easy to find, standard parts.
I hope others find this useful!
The axle of the wheels on the MacLaren Quest is 8mm in diameter which is the same as for standard rollerblade and scooter wheels (inner diameter with ball bearing mounted). This means that you can replace your worn original Quest wheels with scooter wheels by using a standard 8mm ("M8") bolt of 100mm length as an axle. The scooter wheels are not expensive, very durable, and very easy to find in sports/rollerblade/skateboard shops and the bolts can be found in any diy shop.
FYI the steps for each double wheel are:
1: Remove old wheels: First pry off the plastic hubcap, then remove the star nuts with plyers (as mentioned above this requires a bit of time/effort, there are videos online that show how to do it), then the different washers and the wheels.
2: Replace with the scooter wheels: First place the first (outer) scooter wheel on the M8 bolt, then place 3 or 4 washers on top to keep the appropriate space (otherwise the wheels might rub on the frame). Then push the bolt (with the outer wheel and washers on it) through the holder on the Quest, from the outside. Then place 3 or 4 washers for space again, and the second (inner) wheel.
3: Lock the wheels in place: Simply use two M8 nuts (as jam nuts) to lock the wheels in place while leaving them to spin freely. First thread the first nut on the bolt by hand until it is about 1-2 mm from the wheel assembly. Then thread the second nut by hand until it touches the first. At that point you need to tighten the second nut with a spanner/wrench while also holding the first nut with a spanner to prevent it from moving. Check that your wheels spin freely, if not try again while initially leaving a bit more space below the first nut and/or ensuring that the first nut doesn't move when the second is tighten.
Notes:
- Total cost for the four back wheels was around €25, but the set up should last very long (and doesn't depend on MacLaren or anyone stocking specific replacement parts).
- I used 100mm scooter wheels which worked fine. 110mm wheels would probably be OK too.
- With some scooter wheel designs, the Quest braking system may not work anymore.
- Scooter wheels are much more durable and faster than the original Quest wheels but also harder, so it will be bumpier.