Temp sensor

Hi we have a problem with our boiler with it going hot and then cold during showers etc. It also seems to take awhile for the temp to raise. I believe it's the temp sensor from reading posts on here and also speaking to a professional. They guy we spoke to has quoted £130 + VAT to replace the sensor. Is this reasonable as it seems to be a cheap part?

Thanks

Try this link;
http://www.segel.co.uk/index.php?main_page=index&cPath=19_63_85&sort=20a&page=1
DHW sensor is around £30:00p
But it may not be that.
Mike
MGC/H2O
Norfolk