Gavin
The short answer is the most obvious one – you will need a Planet T5 transmitter. These aren't that easy to come by because they are now discontinued. The S/H ones which appear occasionally on Internet forums and on E-Bay are quickly snapped up. It might save you a lot of trouble if you replace the receiver with a different make and use its partner transmitter. 4, 5 and 6 channel 2.4GHz radios are readily available for little more than pensioner-pocket-money. Here's just one I found **LINK** although I can't recommend any particular "budget" radios on the basis of personal ownership.
BTW You'll easily be able to sell on the Planet receiver to defray the cost of a new set.
Dave M
Edited By Dave Milbourn on 23/10/2018 10:13:55