Sorry Stewart - only just seen this! You can play with a levels adjustment or simply Increase the exposure in Affinity by one stop either of which will retrieve the situation without noticeable consequences. It might be different for 3 or 4 stops, though - it depends on your camera model - if the interweb says that it has an 'ISO-less' sensor, life will be easier than otherwise.
To handle this as a batch process, you are limited to (a) using the bundled image managment software that came with your camera (not all have this - you don't say what you have) or (b) usng Adobe Lightroom, which I guess you don't have or (c) using Affinity 2's batch processing function. For this, you need to create a macro (i.e. a repeatable series of commands) To do this, create the macro first using the following steps on a single image:
Load an image (presumably one of the set you wish to affect in this way) and get into 'develop' mode
Open the macro panel (under the menu item 'Window', select the 'Macro' option)
On the Macro panel, click the Start recording icon.
Perform the operations you wish to record (in your case, exposure increase by one stop: Top Menu; 'Layer';'New adjustment Layer';'Exposure') on the image that is loaded. ...
When you are finished recording, click the Stop recording icon.
Click the 'add to library' icon in the macro panel - save to the 'default' category and name it appropriately
So now you can proceed to batch operate that macro..
Under the 'File' menu, with no images open in Affinity, choose 'New batch job'. Then click the 'Add' button at the bottom af the panel that pops up. Select all the files you wish to process; pick your recently-saved macro from the drop-down list under 'Available Macros'; choose the format for the file to be saved under the 'Output' section forget the columns of boxes labelled 'W' and 'H' (unless you want to resize them as well) and click OK. Voila! Affinity runs through all those selected files and saves them with the changes defined by your macro.
Of couse, macros aren't limited to just one action - fill yer boots!