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OK here is the link to a Youtube video showing how to connect the Siemens RAA series thermostat , note that this is connected in heating mode ie Live in To L, Switched Live out from Y1. https://www.youtube.com/watch?v=5hWQHVT7haM
This link takes you to the boiler installation instructions - https://www.worcester-bosch.co.uk/professional/support/documents/greenstar-ri-regular-installation-instructions -- select your model and you then connect up the boiler by removing the link shown and connecting the live side of the link to L on the thermostat and the switched live side of the link to Y1 on the thermostat. - Ian
Copy and paste this link in your browser which takes you to the illustrated parts listing for you model, ~ http://www.neff.co.uk/store/sparepart/finder/detail/S51E50X1GB/14 ~~ go to page five and the area that you need to be looking for water is item 0520 the base.