Quality timber reservoir raised deck kits for the garden or allotment to store & harvest up to 4000 or 8000 litres of rainwater (equivalent to 20 or 40 200ltr water butts) depending on the deck you choose.  
You need to dig a hole in the ground up to 2m x 2m x 1m deep or 2m x 4m x 1m deep if building the double reservoir - drive the metal post anchors into the ground - fit the posts, secure the joists, fit a deckboard to the posts just underneath the joists and use the specially designed pond liner with fleece to create the reservoir.
See the assembly instructions by clicking onto the link by the the deck size. 
This picture shows a 3m x 2.1m reservoir raised deck built for the horticultural channel on Sean's allotment with a 4000 litre reservoir underneath. Click on the link for the final day video. NB some changes have been made to the construction since the video was made

 http://www.youtube.com/watch?v=n060v-vZAxg&feature=player_embedded 
You have to provide the water supply which in most cases will be from the overflow of your nearest water butt using a piece of hose (not supplied). By using one or more DIYdeals solar irrigation kits you can regularly water your plants every 3 hours during the day.
Reservoir raised deck kits include: all necessary deckboards, joists, deck screws, joist hangers, corner brackets, posts, bolts,  pond liner with cover and battens with fixing screws to fit the liner to deckboard below the joists.
Post Anchors with Spikes are included to secure the 1,25m posts & make fixing easier.
A 200mm x 11mm drill bit for the bolts may be purchased if required - see prices link above

All Decking Kit prices now include VAT and delivery to mainland England (excluding Devon & Cornwall), Wales and Scotland up to the M8 corridor including Glasgow & Edinburgh. Delivery usually takes from 1 to 10  working days. Southern Scotland & South West Wales may be up to 3 weeks, but for most of the season we manage within 1 to 10 working days.
 

To place an order or ask any questions simply email diydeals@hotmail.co.uk, call 01457 855259 or text/sms 07866 767863 stating your requirements, your full address including postcode and a contact telephone number for our carrier, who will call you with a delivery day and a 2 hour time slot. 

Payment cash on delivery, credit card, by bank transfer or  via Paypal.
 

Listed below are prices for 3.0m, 4.0m and 5.0m raised decks with reservoirs
Metal Post Anchors with spikes are included in the cost.