Can you not do it yourself, buy a tin of fabsil and coat your tent!!ihave never botherd with ours,and never had any problems with any of our tents. I have always been told, if they don't leak then don't touch them!!
A polyester tent is waterproofed during manufacture, the waterproof laminate being on the inside. It should not leak directly through the material. The added protective coating on the outside causes rain to bead and run off easily so that the outer layers of material don't retain water. This also means it will dry quicker on going home day. These days the outer coating also contains a UV protection.
Over the years and after lots of use the outer coating may gradually wear off and the tent will start to look wet when it rains without beading, but it shouldn't leak.
It can, if you wish, be re-protected with Fabsil or Nikwax which can be purchased in spray cans or painted on with a brush. It can be expensive, Fabsil being about £7.99 for a 600ml spray can and depending on the size of your tent you may need 2 or 3 cans.