ive also got a 56 plate hawkeye sti and experiancing the same issues mate,only my temp gauge did move once,for about a mile (after some hard driving) but soon settled and its never done it since,but my expansion tank was full to brim and spilling out,when cooled down and took my resonater cap off that was very low,so was my rad,when i squeezed the top rad pipe it was doin nothing in the resonater tank,i just topped up and let it run for a bit,like you i was ****ting it that me HG had gone,but no other signs,just like you,and not losing any power,which id expect.so im assuming it was a block(for some wierd reason) yours could be same mate,hopefully anyway,and nothing major..keep us up to date
