Webhoster's /tmp folder

Help, I am stuck and I just don’t have the IT knowledge to understand what to do:
Fasthosts, my web hoster, has been telling me that :

We have suspended evamusby.co.uk as it’s saving too much information to the server’s tmp partition. This is a shared webserver so you should create a tmp directory within your own webspace and save temporary/cached info to that folder, not to the server’s tmp partition.

They later said:

your site was regularly consuming approx 90% of the total tmp partition, which is the reason it was suspended without prior warning. Each time it filled the tmp partition, no other users could write anything to that partition, which can cause their sites to fail. It is designed for temporary files – typically session data, which should be removed shortly after being written, which was not the case for your own site.

Right now your site is using 364 times the next largest user of the tmp partition. I have just deleted all this content, however, it is not practical for our support staff to keep doing this.

Looks like in just one week 900MB of data built up in there.

So I followed Fasthost’s instructions to do this:

To set the tmp directory for wordpress create the following directory in your webspace htdocs/wp-content/tmp and add the following to your WordPress configuration file htdocs/wp-config.php

define( ‘WP_TEMP_DIR’, ABSPATH . ‘wp-content/tmp/’) ;

But a week later I see that my htdocs/wp-content/tmp is still empty, whereas Fasthosts tell me:

Unfortunately we are still seeing a very large amount of files appearing in the web server /tmp directory. An example of these is below. Please look into this further as soon as possible.

/tmp/sess_rj27frr0p97ue8v67j69kadtu7
/tmp/php2qYLFT
/tmp/phpdgX1yC
/tmp/phpeJoMv1
/tmp/phpUo3GM3
/tmp/phpuwQJCe
/tmp/sess_ekp2n4pfp6ir7m04k6ffu49er1
/tmp/phpWpDz52
/tmp/phprcv4Du
/tmp/phpnTXkk2
/tmp/phpfhR2V4

They say the problem is most likely a plugin I’m using, but surely then other users of the plugin would have the same problem? And what would be an effective strategy to guess which plugin?

Any help gratefully received, and please remember I don’t understand much IT-speak.