Thanks all, I ended up with some B&Q tubing intended as part of an
irrigation system 10m was under 5 quid.

For posterity and Google:

*How to drain a Fortic Tank that doesn't have a drain*

Copyright Steve Harris 2004 but placed in the Public Domain - No
warranties. All possible disclaimers apply.

Allow 10 hours - but you won't be working for 10.
You will need a siphon tube of at least 4m - longer if a bath/sink/drain
is further away. I used B&Q "Water Supply Pipe for use with B&Q easy
watering system! 10m long, 7mm diameter.

1. Start with the tank full
2. Stop cold water coming in. Careful application of elastic cord on the
ball cock can avoid the need to turn off the rest of the system
3. Fill the siphon tube with water. Block one end with a lump of blue
tack and the other end with a finger. (Filling tip: Put tube in a bath,
stuff one end up the cold tap, turn on and wait for a steady stream at
the far end)
4. Plunge your finger, tube and hand into the water in the top tank.
Take your finger off the tube end but ensure the tube end is kept below
the water.
5. Feed the tube into the pipe joining the two tanks. On my tank, it's
angled favourably. Keep feeding, you will probably be able to tell when
it hits the exit and enters the bottom tank.
6. Feed in another 6" of tube for luck
7. Unblock the far end of the tube and check for a steady flow. Have a
cup of tea and come back to make sure it's still flowing.
8. Turn on hot taps etc to assist draining
9. Do something else for about 9 hours - that's how long mine took to
drain.
10. Suggest you leave the tube in place (bunged!) until you are sure you
won't need it again.

Steve