The total allowable tax free lump sum in a public sector scheme is 1.5 times your pensionable salary at retirement (that is full time equivalent salary plus any pensionable allowances in your final three years).
If you will have full 40 years service at retirement then your Occupational Scheme should provide this amount and you cannot take anything further tax free from your AVC. If you will have less than 40 years service (but greater than 20, again full time equivalent) then you can top up the tax free lump sum to this amount. I see you are working part time, so it is likely that you will have scope to top up the lump sum.
I don't know about AVC comparative charges. As you are part time this will also limit the size of annual AVC contributions you can make in order to avail of tax relief.