I had this problem a few years ago. First I tried a big trolly jack under it, but that just lifted the front of the vehicle up. I got a heavy chain, placed it over the top of the crossmember and then under the jack, shackled the two ends together and, placed a short length of scaffold tube between the jack cup and the bottom of the relay. then pumped up the jack. after a fair bit of pressure had built up, bang, the relay released and I could easily jack it all the way out.
Regarding Ruxy's post about the inferior pattern parts, I had one snap just above the drop arm. It was made by a large British manufacturer. Luckily it was in my off roader, and I wasn't travelling fast so there was no mishap. I had to get a replacement and change it in the bog the Land Rover was sitting in, getting bitten to pieces by clegs (horseflies) that there was a bumper crop of that summer. Happily the relay had only been in for a year or so and it came out easily. Put lots of copper grease in when you fit the new one.
Hope this helps, Davie.