Triton Cara 8.5Kw on offer at B&Q if that's any good to you mate.
<a class="postlink" href="http://www.diy.com/nav/rooms/bathrooms/showering/showers/electric-showers/Triton-Cara-Electric-Shower-White-8-5kw-10517829?skuId=11008537" onclick="window.open(this.href);return false;">http://www.diy.com/nav/rooms/bathrooms/ ... d=11008537</a>
Something must be wrong with previous installation or the unit itself if it only lasted 2 years. I would expect one to last 8-10 years with scheduled cleaning of the shower head and inlet filter.
Never had an issue with Mira or Triton showers either, not had a Gainsborough though.