attended a career sharing before with ES:

typically 3 stages:

i. Written exam - Macro, Micro and Econometrics + 2 essays. Total time: 2.5hrs
ii. Suitability screening/ interview by junior economist
iii. Panel interview with directors and chief economist (Hd)

All 3 stages are very technical in terms of economic modelling and economic intuition. Be prepare for an environment like thesis defence (that was what i was told).

First few years in MTI, subsequent can be posted out to do analytics or quant work. The ES is like a training programme and they second economist all around the civil service and SBs.

Salary wise is good (as compared to the rest of CS) because its a specialised scheme as they have their own scheme. It's not easy to get in though, and minimum requirement is a 2:1 Econs degree. Heard there are a lot of scholars and a lot of those are from top universities in the UK (LSE, Oxbridge, UCL etc) - so some people can be quite elitist to some extent.

Good luck if you are applying that!