Wifi works fine round the house but I would like to beam it to a Summer house some 50 yards away down the garden with a directional antenna. Can I just use a directional antenna inplace of one of the three supplied antenna or is that not going to work as it transmits and receives using separate antenna. Quite happy to drop to g from n if that makes any difference.

Take a look this guide if helps you
http://www.draytek.co.uk/products/accessories/aerials

If the 2710 is nearby to the window which is facing the garden will work very good with the ANT-2510 ...

But if the antenna or any other external antenna will be placed indoor or the other side of the garden or in general the cable is very long from the 2710, i would not expect to go very far out of the house ... because ...
There is a case if you use any directional antenna connected at the 2710, if you use long cable more than 2-3 meters to start loosing some signal ... perhaps an other member can have opinion on this

The Draytek antenna ANT 2510 it seems has not the option to get ethernet or RJ45 cat5e cable, apart the CAB2 type cable where if is too long can loose signal as mentioned

:arrow: If the 2710 is at the other side of the garden or not facing the window garden, i would get an AP800 ... then cable RJ45 as long you need from the lan of the 2710 to incoming wan of the AP800 (can fix the cable side by side the wall) & put the AP by the garden some where where rain will not get it)
Here how to set it ( i prefer to close the DHCP server configuration at step 2)
http://www.draytek.com/index.php?option=com_k2&view=item&id=2777:ap800-how-to-use-universal-repeater-mode&Itemid=293&lang=en

So summary as explained if the 2710 is near by the window facing the garden is fine ... is far from the window which is facing the garden i would go the other choice with an other AP