Have I been sandboxed?

Hi. I'm hoping you can shed some light on a problem I'm having.

For around the last 6 months, my website limotek.co.uk ranked well for particular keywords but as of this morning, the site seems to have dropped on average 5 pages for each keyword. Have I been penalised / sandboxed by Google? The table below shows some examples of where the site was yesterday and now the position is today.

Keyword | Yesterday | Today
Limo Hire | 4 | 68
Limousine Hire | 5 | 63
Limo Hire London | 7 | 68
Limo Hire Newcastle | 3 | 38
Limo Hire Liverpool | 3 | 55
Limo Hire Birmingham | 25 | 66
Limo Hire Essex | 16 | 43
Limo Hire Surrey | 27 | 63
Limo Hire Manchester | 11 | 64
Limo Hire Glasgow | 5 | 48

I'm thinking this could be a sandbox/penalty as all key phrases have dropped by a similar amount. The only change I have made to the site in the last month has been to amend the address in the footer (done yesterday) as we have moved offices although I'm thinking this is too recent and minor to be the reason.

In terms of link building over the last few months, this has been article, blog and forum posting as well as a handful of link exchanges. I have never paid for links.

Another interesting point to note is that a major competitor has also experienced the same problem and has had a lot of their websites dropped by a similar amount which is why I'm thinking sandbox/penalty.

I'd be grateful for any feedback you can give.