Hi Dominic, you need to contact MTN support for them to reset the MoMo pin for you.
You can reach out to them via any of these channels.
1. MyMTN App
2. Facebook: @MTNLoaded
3. Instagram: @MTNNG
4. Live Chat: http://bit.ly/2OC5bHz
5. Email: customercareng@mtn.com
6 .WhatsApp: 09033000001