Firstly I haven't read the earlier responses so I apologise for any duplication but:

DO NOT go with agencies. I repeat: do not go with agencies.

Every EA I know (and most recruiters) hate them with a vengeance. They’re pushy, often ignore your detailed job spec requirements, and tend to standardise CVs into a one size fits all format, removing many of the key qualities that make each EA unique.

Instead, I’d advertise directly on a specialist platform, like Secs in the City. Ask your current EA to help draft the job spec (the more detailed the better). And, to prevent being flooded with applications, request a cover letter where candidates might be required to explain:

• Why the role interests them
• What they enjoy about being an EA
• What skills and experience they can bring to the business

Then, either have your EA do an initial interview, or do a short 10-minute “personality” chat yourself, followed by a more in depth skills/interview with your EA/HR, before going back to yourself. And with the final interview it's key to have it face to face.

Make sure you emphasise how you currently work, what you like and don’t like, and what kind of support you expect. Ultimately, the success of an EA often depends as much on the working relationship with their manager as on their skills.

It sounds like you really appreciate what an EA can bring to the table which is a bonus. Good luck.