According to WotSat (yes I know it is not very accurate) Telstar 12 on 15W should be possible with a 90cm dish from the south of the UK. I can't get anything but I am (fairly) sure I have my setup correctly aligned.
First, Telstar - like most satellites - has multiple beams, aimed at different parts of the earth's surface. You need to determine the beam that carries the transponders you are interested in.
Check out Lyngsat for this: http://www.lyngsat.com/t12.shtml
Let's concentrate on the main "Europe" beam. This carries transponders which include channels such as Maharishi Open University.
Here's its footprint: http://www.loralskynet.com/images/fleet/telstar12/telstar12_euro_ku_pic.gif
You'll see that most of the UK falls in the region of signal strength marked 48 or 50 (dbW)
Here's a table that converts bewteen such strength figures and the recommended dish size:
You will see that for 48dbW a 70cm dish should be ok.
So check your receiver's transponder settings against the data at that Lyngsat site, and check the dish alignment. Also note that not all channels transmit at all times in the day. And many of the channels on Telstar are PowerVu which some receivers will not even register.