I'm surprised every guide book on Thailand doesn't forewarn. It's a very serious issue and every single year of the 15 I have lived here it's been the same but this year was certainly the worst so my advice would be to look elsewhere.

The best time to visit the north is undoubtedly during the rainy season. Places such as Pai, Mae Hong Son, Chiang Mai and Chiang Rai are at their verdant best and typical rain pattern is torrential downpours for less than an hour then blue sky and clean air so easy to stay dry.

HaGiang Province is superb - scenery is spectacular & there are many ethnic minorities to see. Ninh Binh is also well worth a visit. We did Hanoi, Mai Chau, HaGiang Province and Halong Bay on our first trip to Vietnam. We included Ninh Binh (and surrounding sights) on our second trip to Vietnam when we went from Hanoi to Ho Chi Minh City and several places in between including Ninh Binh and also the Mekong Delta towns.
We (four of us) had a driver/guide with a SUV for 8 days to go from Hanoi to HaGiang Province, Babe Lake and back to Hanoi on our first trip.

there is a recent similar thread here which also contains a link to a downloadable 280 page ebook on the National Parks
https://www.lonelyplanet.com/thorntree/forums/asia-thailand/thailand/thailand-beach-camping

April/May isn't the ideal time, as it is the start of the rainy season on the Andaman Sea. It is not the end of the world, but you might get rained on in May. If you don't mind a bit of rain, it shouldn't be a problem.

The Gulf islands Tao/Pha Ngan and Samui would be better weather-wise, but would still be very hot and don't have National Parks (except Ang Thong, which has very little little trekking).

If you have a desire, then do it. It is not necessary that you depend on the opinions of people. Everyone has his own! Or do you want a real tip to read ???

Just want to know if it's overrun with tourists like Pai. I could go to 500 different places but don't have time.

I can't help with all of it but some points that might be able to help.

• One World airline alliance does a round the world ticket but you must go one way only (east or west) if you look at their website it explains it and allows you to plan. I would guess other airline alliances do the same thing.
• From London you can easily fly to Singapore or Bangkok. I would recommend flying to Bangkok and then travelling overland to Cambodia (maybe Laos?) and Vietnam. If you finish at Hanoi you can then Fly onto Singapore and then fly to Indonesia. I don't think there are direct flights Vietnam to Indonesia. Flights in SE Asia are very reasonable.
• I personally haven't been to the Pacific Islands but my girlfriend recommends Tonga if that helps!!

The problem there is I want to travel overland from Bangkok to Singapore (or vice versa) as I want to visit Phuket/Phi Phi, Kuala Lumpur etc. I was thinking flying to Vietnam from UK and just work my way to Singapore - but I don't want to do too much overland travel. I appreciate I may have to drop a few places.

Thanks all! I visited Snow & Rock last night and had a look at the Osprey Farpoint 40, before heading home to try a test pack with the existing Kestrel. What I learnt is... wow, I need to bring less stuff!

Fully loaded, as comfy as the Kestrel is, it's seemed more suited to hikes, so I ended up ordering the Farpoint 40 while it's on offer at Wiggle. I'm going to do as many others have suggested and pack for one week (rather than 2), and then just use laundry services.

I'd have gotten a bigger pack, however we're thinking about hiring mopeds and riding from Chiang Mai to Pai (I gather there's an amazing route along there), so I don't want to be too loaded up. That's also one of the reasons suitcases were a no-go for us. Bit expensive buying another bag but as this fits in most flights, I'll get years of use out of it :-)