I've got two transmitters, two separate antennas on the roof - located at either end of the band so plenty of separation, both have band pass cavity filters on them. When both are powered on, the digital TV signal goes to zero, however its ok with either one of the transmitters powered on but not both. The TV antenna is located about 5m away from the transmitter antenna's. I've also sampled the RF going out of each transmitter into the analyser and there's no intermods being generated from the transmitters, so i'm assuming its coming down to one thing and that's intermods being generated in the TV tuner.

Have you checked what the level of RF is around the TV? If the two signals are entering the TV through it's case, they can mix in any semiconductor junction inside and cause problems. It might be in the tuner itself, but can't see much signal getting in through the TV antenna. You could test that by placing a high pass filter in line with the TV antenna cable. The signals my be getting into the TV via the shield of the co-ax cable too. Someone one told me that RF is like dog turd, it manages to spread everywhere if it can.

One thing I forgot to mention, one of the transmitters is located directly under the TV, the other is down the other end of the house - I might try relocating the transmitter currently under the TV and see how that goes. Other than that, if it doesn't solve the issue i'll try a high pass filter on the TV or a couple stubs to reject both FM carriers - the latter might be easier as its just lengths of 1/4wave coax open ended.

ssuming your trnasmissions are pure, could be one of two possible mechanisms:

Intermod - especially probable if the TV aerial has an amplifier - they are really easily overloaded, producing sprogs at multiple frequencies

Blocking - even if there are no spurious signals generated, a common problem is blocking - the front end of the TV receiver might be wide enough that it sees the strong TX signal and turns down its AGC or overloads its first amplfier.

Both could be mitigated by having a filter in the TV aerial lead, but that may not be possible if you have an amplifier built into the antenna. An attentautior can also help, especially with intermodulation because a classic indication of inter,mods is that if you reduce signals by 3dB, the interference reduces by much more - say 9dB, maybe 20dB.

Another possibility - have the FM TX at right angles instead of behind the TV yagi - you might get more rejection.

Try a "braid breaker" high pass filter on the TV antenna lead, a man such as yourself should be able to make one of these in 5 minutes, so its worth an experiment. It does sound like the FM signals are swamping the TV amplifier alright. Here is a link for a 6 metre version - you may need to take a turn or 2 off the coils...