I want to be able to connect a second tv in HD located in a another room to a humax pvr.
It is not easy to install a HDMI cable and it would need to be a 20metre cable.

Whats the cheapest and best alternative?

I read somewhere about connecting two connect 2 humax boxes via cat5e, but when I search now I cannot find anything threads.

options appreciated.

Which model of Humax PVR ?

I haven't purchased yet, but looking at a youview DTR-T1010 or freesat HDR-1000s so connection options may influence the decision

Neither of those boxes has a DLNA server so that limit your options. By the way I wouldn't recommend the DTR-T1010 if you decide on YouView as reliability is not good; you might look at the later DTR-T2000.

I bought a BT DTR-2100 which I understand is the same as DTR-2000.

Could I connect it to a second DTR-2100 box via cat 5 and be able to access the recordings on the other box?

I have found some HDMI to Cat 5 extenders are now available at £35 ish would this be more reliable/better than connecting a 10metre HDMI to mini hdmi cable with an adapter to a 12.5 metre HDMI to give 22.5metre overall?

HDMI to CAT5 is probably better over longer distances than direct HDMI to HDMI and probably even better than bodging two cables together!

I installed a 15m cable to feed a meeting room projector at work and that works perfectly. Not sure I would want to go much above 20m HDMI to HDMI though.

Plus of course with CAT 5 you could be handling thinner cables and they can be easier to poke through holes in walls (even more so if you fix your own plugs on the end so just need clearance for the wire).