Overview
ClubSpark is provided by the LTA. An LTA user id and a ClubSpark user id are the same thing. We will call it the LTA id …
Existing club members should already have an LTA id. Going back a few years, it used to be possible to log into ClubSpark with various ids, such as Facebook, Google, LTA, etc. By now, everyone should have moved over to using their LTA id.
If you are joining the club for the first time, you may not have an LTA id. If so, when prompted to login to ClubSpark, there should be a link for you to register. Something like this …

If you ‘click here’ then you get something like this:

You click on the LTA button, as requested. You should see something like this:

And after all that, you click on ‘Sign up for free’ and get where you want to be…

Fill all this stuff in, and click on ‘Join Now’ at the bottom of the screen.

Find the email, click on the required verification link, and you should get confirmation that the verification has been successful, and a nice button for you to click, saying ‘Back to login’. This takes you back here – but now you have your login details.

So you put your details in, hoping that is the end of the matter, but no. You then have to go through a few more pages as the LTA attempts to ‘find the right Advantage membership for you’.
Tell them you are a Player, subscribed to a Venue, and choose Thornbury Lawn Tennis Club, Mundy Playing Fields from the list of venues. No need to part with any cash (unless you really want to) and you should end up with Play+ membership. Congratulations!