Ranking analysis

I've been given a set of data to analyse and would like some advice on how to manage the ranking data.

What I've been give: service users where given a list of services (e.g. out of school activities) and where asked to rank them in terms of how important they are to them. They were given 20 service to rank. Some have given ordinal rankings (1,2,3), others have used dense ranking (e.g. 1,2,2,2,3). Some have ranked the full 20 services, whilst others have missed some our and ranked 15 services. I have over 750 responses.

Would appreciate some advice on what I can and cannot do.

My initial thoughts are are as follows

1) Where service users have been consistant and fed back on all 20 services, calculate an average ranking: take each service add up the different rankings (e.g. 1,4,17,20), and divide by total rankings (e.g. 20). Is this allowed?

2) For all responses, divide each person's ranking into quartiles and then for each service count how many times it shows up in each quartile.

thoughts on the above would be welcomed.

thanks in advance