You can more or less fit what you like whether it is a single din or double din unit. Fascia plates are available easily off ebay for single din units which can also come with the wiring adaptor which will make it a plug and play unit. search ebay for a Rover 75 single or 1 Din fascia and if you search the forums for single din you will find many mods members have done. If your budget allows the Chinese units with 7" screens are very popular and give you satnav, freeview, DVD, reverse camera etc. You'd be looking at £300 ish and are an easy install.
Here is a fascia :
http://cgi.ebay.co.uk/ROVER-75-CD-FI...ht_1731wt_1165
This thread may help :
http://www.the75andztclub.co.uk/foru...ad.php?t=45427