How much should I pay

[Tlkamk]

For catering for around  40 people good quality party food?

[Pugwash]

Depending on what type of food - hot/cold, buffet/sit down I would suggest at least £20 plus a head probably more.  You could probably get a better idea by googling a few caterers.

[Sue]

You could try asking  Suzanne James?

She is based in Barry Road.

https://www.suzannejames.co.uk/

This isn't a recommendation, as I have never used her,  but she is well established in SE22 and would probably be able to give you a good idea of cost?

She has examples of various kinds of food for private parties on her website.