Hello madcabs,
Thank you for posting to Apple Support Communities.
We understand you're unable to reply to a group iMessage, yet you can see the conversation.
Let's start by suggesting you restart your iPhone:
- Press and hold either volume button and the side button until the power-off slider appears.
- Drag the slider, then wait 30 seconds for your device to turn off.
- To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
Then, test to see if the issue continues.
If the issue is still there, make sure that your iPhone is updated to the latest version of iOS. Before updating, we recommend backing up your iPhone first. These articles can be of help. When you back up, you can back up using a Mac or PC rather than iCloud if you choose.
How to back up your iPhone, iPad, and iPod touch
Update iOS on iPhone
That should help. Let us know if you have any questions.
Take care.