Hi Beesmummy, welcome to Mumsnet!
OK, I think you may be looking for the impossible!
I'm afraid something with big wheels which can cope with off roading and beaches will not be lightweight. Personally, I prefer to push a buggy with big air tyres and a swivel front wheel, I just find it easier. The compromise is that it is heavier to lift in and out of the car but worth it IMO.
With a 17 month age gap your Phil & Teds may just be the answer. Two children in a single sized buggy, Its as compact as you can get.
If you plan to use a carrier for the new baby then you could use your lightweight buggy for Bee or invest in a really good umbrella fold like a Maclaren Techno which will fully recline for the baby or toddler depending on what you're doing (Bee may fancy walking for a while so the baby can have a ride and give you a break from carrying). Fairly compact and manageable for the car boot, great around town. This does not solve your off road walking problem though.
If Bee is a really keen walker you could get away with a buggy board. She could hop on or off when necessary.
I'm not terribly keen on the Micralite. The new 'Toro' seat unit is far better than the old 'Fastfold' but whole thing is a bit flimsy. It has never felt very stable to me, and has always worried me a little.
Now my big recommendation is the Mountain Buggy. I know it is fairly big and heavy to lift (just something you must accept) it does everything else you need it to do. It is one of the very few buggies strong enough to take a Buggy Board without invalidating the warranty. It has a very generous and comfortable fully reclinable seat unit suitable for both toddler and newborn napping, therefore a long life, it won't be outgrown too soon. Mountain Buggies also hold their value fantastically well so you could get a decent return on eBay when you're done with it.
HTH