X first hides the tandem and side-car in the lane, close to his lair.

..... The deep sandstone cutting, its hedges grown together across the top, is still there; anyone who wishes can dive under the sentinel thorns at the entrance and push his way through .....
..... I worked the side-car into the thicket and deposited it in the first bare stretch of lane, where the foliage overhead was so thick that nothing grew but ferns.

After constructing his lair X has second thoughts, because:

The tandem bicycle could not be seen. I took it apart and propped the pieces against the bank, covering them with a mass of dead vegetation. The side-car was a continual nuisance. I couldn't bury it or take it to bits, and the bright aluminium shone through the brushwood I heaped upon it.
..... Eventually I had to spend a night tearing down my defences in order to get the thing out of the lane, and half wheeling, half carrying it down to the vale. [the Marshwood vale?]
..... Finally I chucked it into a sheltered stream, hoping that the action of water would destroy it; I couldn't.

After visiting Lyme Regis to collect a letter from his solicitor, X is pursued by the police, goes on the run, briefly loses his bearings at the Eggardon Hill fort and eventually heads back towards his lair. However:

..... As I dodged and darted home from lane to lane and farmhouse to farmhouse I couldn't get the side-car out of my head. I wanted to know if it had been disturbed. Should police have found it, and taken it from the stream for identification ..... and search the country where I really was.
..... Although it was only a field away from a well-frequented by-road, the side-car was in a safe place: a muddy little stream flowing between deep banks with the hawthorn arching overhead.
..... I entered the water at a cattle wallow, plunging up to my knees in mire, and forced my way under the hawthorn.
..... At last I saw it, a faint white bulk in the darkness canted up against a bank of rushes where the stream widened.
..... I was leaning over the side-car when a voice quite softly called my name.

At which point X is shot!

So where was the side-car? Given that X had manhandled it down into the vale it from the lane in which his lair was situated, it cannot have been far away.

Conclusions

The nearest streams to the putative location of the lair in or near Bell Flower plot are those:

(a) flowing from Denhay Rookery towards Broadoak Road
(b) from Jan's Hill and Doctor's Copse towards Lower Moorbath Farm,
(c) from Axen Copse flowing into the River Simene at Bishay Farm or
(d) rising from the spring in Henwood Copse.

All four of these are 'only a field away from a well-frequented by-road', either Broadoak Road or the road from Bridport to Dottery, and are close to the direct route to Symondsbury from Eggardon Hill. On balance I favour the first or second of these because they are closest to X's lair and, indeed, as other correspondents have pointed out, the paths down from Denhay Cross towards Denhay Rookery or leading off Coppet Hill are good alternative possibilities for the location of X's den. In the latter case the stream could well be that from the pond near to Cuthay Farm and flowing towards Blackmore Farm. X has just been shot in the chest and doesn't know how serious his injuries are. He wouldn't want to go far back to his den before checking the damage.