What is an "ERU"? I am presuming you mean an Emergency Services Vehicle?
If that is the case, then sorry, there is no requirement for any station to be accessible as such, and I am sure we can all cite plenty that aren't. (EG: Dovey Junction). If there was an emergency that required immediate assistance then other means of access will be used, from driving across land not belonging to the railway, to making arrangements to get the train to a accessible point of access like a station, or level crossing.