I’ve been working at a start-up for 4 years.
I earn a decent wage however I’ve been on the same salary since I started and it’s come to light through chats with colleagues I must be one of the rare few that has never requested (and received) a salary increase. Before this I worked in public sector roles for 10+ years where there were no pay increments beyond the annual £1k as you moved up the banding, as a result, I’ve never requested a pay review in my life and I’m nearly 40 
I'm not really looking for a promotion, I’m happy in my current role however the role has expanded and responsibilities increased as is normal when you become more familiar with a company and its processes. I have no idea what the role is “worth” now though. Benchmarking my salary against other job ads out there and Glassdoor I’m already at the higher end for my title in this field, so I don’t have much evidence to put forward in that respect.
I’m not looking for a substantial increase but I don’t want to leave myself behind. I’m a FT working mum and I focus on the flexibility the job gives me but now I feel like I’m losing sight of my value to them?
All of my appraisals have been v.positive and I've been described as the glue that holds us all together aka I often pick up others' work (especially mundane admin tasks that don't really fall within anyone's set remit).
I was going to start creating my pitch tonight, so far I plan to include:
My responsibilities now vs when I started, and any impact I’ve made with figures to back this up where possible.
Put my earnings into an inflation calculator to compare real-world differences between now vs 4 years ago.
I am stumped as to what figure/range to put to them though. I’d rather be proactive and name a figure than ask them to give me an offer? Maybe double what comes out of the inflation calculator and expect them to haggle down?
Grateful for any tips or experiences anyone can share.