Hi - this is NOT what's causing the issue (unless I'm missing something?)as the setting under 'maximum memory wordfence can use' is 256Mb - i.e. twice what you have quoted. I haven't adjusted the 'PHP.ini file' because a) the maximum memory has been 256Mb since the website was first created and this hasn't been an issue before and b) I haven't a clue what the PHP.ini file is/how to adjust it/why I should need to...
I changed the update interval because it was a suggested fix. But it didn't work. The issue I have hasn't been an issue prior to updating to the latest release, so the natural assumption is that the 'bug' was introduced with the latest release, which is what Stever 22 seems to be saying too and within exactly the same time period. If there's a way I can rewind to the previous (and, on my site at least, stable) version of Wordfence I'll try that and wait to update manually until the next release in the hope that whatever's causing the problem is resolved.
Any other suggestions greatly appreciated, but for now i've had to deactivate wordfence and install a different security plug in. It's either that, or I have no site.