I have a heavy wetter too, he pees right through disposables or any 'day type' nappy.
Since I've started using dedicated night nappies though I've had no problems or leaks which has been great, they honestly last longer than sposies.
One of my fav night nappies is a Minki Huggle - they're really reasonably price about 9 pounds new and 4-5 pounds in good second hand condition. They also have a large pocket for stuffing inserts, I use a bamboo terry square folded, or a small blueberry combo insert or sometimes a Greenkids hemp insert which makes for a bulky nappy but is super super absorbant.
Ella's House Bumhuggers have been good too - they are made of hemp, with a good pocket for stuffing and they have a fleece inner which keeps bums nice and dry. Again you can pick these up for around 5 pounds in good condition second hand.
Easy Peasy Bambeasys are good too when adequetly boosted - I like to use these with bamboo squares and a fleece wrap.
People rave about WeeNotions Night Notions - I haven't tried them yet but have one on the way! They are expensive if you get them with embroidery etc but if you choose a plain bamboo towelling one, you can get them for around 12.50 new or less if you buy second hand.
Holden's Landing and Upsy Daisy also do popular night nappies, but if cost is an issue, they are expensive particuarly the HLs.
A fleece or wool wrap will also help night nappies last longer as moisture evaporates.
If you need some more boosters for your night nappies, this ebay seller: http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... 0459355612
has Tots Bots bamboo towels for 2 pounds each with free postage and packaging - it's the same material they use to make bamboozles. They're so cheap I've bought a load to cut up and stuff nappies with. If you get on with microfibre you could try microfibre cleaning cloths from Tesco (about a pound for a pack of 3)
Also babykind.co.uk often has cheap inserts and boosters in their bargain section.
Most people have to experiment to find a combination that works well for them at nights but I'm sure there's a night nappy out that will be perfect for your little one! And if you buy preloved and sell on, you have nothing to lose XX