I'm making a vegetation survey map using the UK phase 1 survey colour codes. The code for marshy grassland, for example, is orange overlain with purple diagonal lines. For the moment I've created separate polygon shapefiles in QGIS for the solid orange and then drawn by hand an additional polygon over the top of it to provide the purple diagonal overlay. Is it possible to have a solid fill and diagonal lines (or other pattern) simultaneously in the same shapefile?
It is possible to have a fill color and some diagonal lines at the same time in a single shapefile. You have a button to add a symbol layer in the style properties of your layer.
Building on from the answer provided by @Alkwts which shows clearly how to add styles, you can copy/paste existing polygons:
If you toggle the Toggle Editing button (icon in the left red circle), you can then select the polygon you want (icon in the right red circle). Once you selected the polygon, click on the Copy and then Paste icons (middle red circle). Your polygon is copied.
Now you want to move the copied feature, so click on the Move Feature icon, select the same polygon and drag it to your desired place.
This next part is if you want to differentiate between polygons as this will help allow you to have different styles for different polygons. Edit the attributes of your layer (right-click the layer name and select Open Attribute Table), select Add Column and enter a name for the column and add a value/string etc to differentiate between the 2 polygons.
The Styling has already been clearly answered by @Alkwts but to give different polygons a unique style, select Rule-based, add a filter which would include the new column name and the value/string you added for the polygon. This means that all polygons with the same value you inserted as a filter will all look like that specific style.
And you can create more styles for more polygons.