Sounds like it's either the relay or thermal cut out. This shower comes in several different power ratings.
Firstly try cleaning the inlet filter, see page 17
<a class="postlink" href="http://secure.tritonshowers.co.uk/media/custom/upload/File-1359718941.pdf" onclick="window.open(this.href);return false;">http://secure.tritonshowers.co.uk/media ... 718941.pdf</a>
The relay might be getting old
<a class="postlink" href="http://www.showerdoc.com/shower-spares" onclick="window.open(this.href);return false;">http://www.showerdoc.com/shower-spares</a>
Replacement may be a more economical option.
For a new shower:
Look at the cable going into the shower, if it's 6mm2 only buy a shower rated up to 8kW. If it's 10 or 16mm2 you can buy a shower rated up to 10kW.
If you want to put a new shower in it must now be done by a part P qualified electrician. They will also check out that you have the correct protection at the consumer unit end. I'm only telling you what you can buy as a replacement. If your shower was not powerful enough before and you want one more than 8kW you will more than likely have to get the cable and protection circuits upgraded as well.