How can I fix my Mira advance alt thermostatic shower?

Manual here:

http://www.plumbcenter.co.uk/wcsstore7.00.816.246/ExtendedSitesCatalogAssetStore/images/products/AssetPush/DTP_AssetPushHighRes/std.lang.all/ti/on/Mira_AdvanceATL_Installation.pdf

Check this is definitely for your shower.

I can't reconcile your description of "blue warning light" with anything in the manual. Check from Page 20 onwards and see if that gives you any clue.

You may also get some info to questions posetd on this site:

https://www.showerdoc.com/mira-advance-atl-post-2003-shower-spares

Scroll down to the bottom of that page. You may find someone has asked about something similar already.

Acp
September 2017