As someone who has just put all new lights in an RS I would suggest you get along to your local auto sparky and have him order the correct parts. Narva still do them but there are probably others as well.
You cant use a cheapy generic light as the buckets the RS lights sit in are all different and the correct lamps have the tags in the correct place to suit. ie the main lamps and the driving (high beam) lamps are paired with the tags on the back of the lamp facing the same way The correct part no also has the required seperate park light holder that pushes in the back of the main beams.
I still have the boxes here somewhere if you need the part numbers.
Cheers
Shaun B