Well after much prompting from Gary (Granite) I finally tried out Photobox (www.photobox.co.uk) as a supplier for online prints.
I took a favorite shot, prepared it as I would for printing at A4 size on my Epson 890 and made a print on Epson premium gloss. I then sent the same file (uncompressed jpg) to Photobox and ordered A4 and A2 size prints - the file I sent was 2568 pixels on the longest dimension ... (Photobox recommend 2400+ for A2).
The results were interesting. At first glance both the Photobox prints were excellent, however, comparing my Epson generated A4 with the Photobox A4 I didn't see a big difference in quality. On the whole, the Photobox print had more natural colours, but less contrast. The Epson print had more punch - arguably too much. Surprisingly the Epson print was slightly sharper than the Photobox. All in all, I would see no reason to use Photobox for work at this size ... but it has provided me with a useful reference for fine tuning my colour settings.
The A2 image however, tells a different story - it is almost as good as the A4, despite being 4x the size. Colour & contrast are nearly identical, and there is
negligable noise/grain. The image is slightly less sharp, but only slightly and this difference is only visible on close inspection.
In short, while both Photobox prints are excellent, I think I'm happier with my own printer for sizes up to A4 in that it is both cheaper per print and gives me total control over the final image.
But for large prints, Photobox sure impresses!