Hi, Rozzie, and welcome to WriteWords
I think you've definitely come to the right place; there are published authors and struggling to be published writers here, who've all been through the submissions mill and can give you advice.
First off, as a trial member, you can dip your toe in the water by joining one writing group and uploading one piece of work. If you are gearing up to resubmit to agents then it's a toss up between joining one of writing groups and uploading the first couple of thousand words of your manuscript for feedback. Or you could join the Synopsis and Outline group and upload your synopsis for feedback, (and where you'll also discover the difference between a Query letter and a cover letter).
Since UK agents usually read the submitted chapters before the synopsis, it may be best to upload the opening of your mss to see if there are any problems in writing technique that are holding you back. Then, if you like us and decide to become a member, you can join as many groups as you want, including the Synopsis group and get some more feedback before sending out your revised submission.
If you need any help navigating the site just give us a shout.
- NaomiM