From the UK, or indeed anywhere in the northern hemisphere, April and May usually represents the best time to see Mercury in the evening sky. The steep angle of the ecliptic (which approximately represents the paths along which the planets travel) does its best to allow Mercury to be seen in a reasonably dark sky, but it is seldom easy to spot, so a guide like Venus or the Moon is too good an opportunity to miss.
Mercury takes only 88 days to go round the Sun so the movement from night to night is quite obvious when compared with the much slower moving Venus. It passed behind the Sun on 14 March swinging eastwards out into the evening sky, reaching a maximum distance of 19 degrees on 8 April at which point it was just 3 degrees from Venus.
In the following days Mercury progressively presented more of its dark side towards us, so even though the distance to it dimished, the brightness dropped rapidly. Both the inner planets Mercury and Venus show phases like the Moon and It is a fact that when Mercury is on the far side of its orbit, and so furthest from us, it looks the fullest and and brightest. This Sky and Telescope article has a diagram that expains the situation or use the orbit simulator at this Venus Transit website to try it for yourself (plug the planet Mercury, time interval Day, Speed 3 then press the Run/Stop button).
An arctic airstream over the UK brought a good number of reasonably clear evenings, plus also cool temperatures and an ash cloud from an erupting Icelandic volcano - Eyafjallajokull. Thus from the following series of pictures it is possible to see the changes in the position and brightness of Mercury, though unfortunately no evidence of the ash cloud. The sunsets were pleasant enough but not spectacular.
Mercury's closest approach to Venus - in the sky, though not actually in space - and also the furthest from the Sun at 19 degrees. Easily visible with the naked eye and capable of being seen in almost a dark sky an hour after sunset, though if you waited too long it would disappear into the murk near the horizon.
Venus and Mercury side by side on at 1946UT (2046BST) on 8 April 2010.
Venus is at magnitude -3.8 while Mercury was -0.0.
Four days later you can see that Mercury is lower than Venus as it begins its journey back in towards the Sun as it swings around towards us...
Venus and Mercury on a clear evening on the North Downs at 1954UT (2054BST) on 12 April 2010.
Venus remains unchanged at magnitude -3.8 while Mercury has faded to +1.7, and unless you were looking for it is unlikely that it would be noticed.
Three days later the exremely thin 1 day old crescent Moon joins the party. Here it is just about to set over the North Downs.
Venus and Mercury joined by a 1 day old Moon just setting over the North Downs on 15 Apr 2010.
Mercury has faded further to magnitude +1.2 and was now becoming a struggle to see without the aid of binoculars. Roll the mouse over the picture to see it without captions..
The following day I took this picture over the pretty village of Stanion in Northamptonshire. The 2 day old Moon has now passed Venus but but where is Mercury? Roll your mouse over the picture to find out! It was by now impossible to see without binoculars and was the last time I saw it this season.
Venus and Mercury overtaken by a 2 day old Moon on 16 Apr 2010.
Mercury has faded further to magnitude +1.4 and I did not see it with the naked eye at all, only with binoculars.